Add send transaction with external sign

pom.xml

@@ -18,7 +18,7 @@
         <connection>scm:git:git://github.com/skynetcap/solanaj.git</connection>
         <developerConnection>scm:git:ssh://github.com/skynetcap/solanaj.git</developerConnection>
         <url>https://github.com/skynetcap/solanaj/tree/main</url>
-    </scm>  
+    </scm>
     <developers>
         <developer>
             <name>Michael Morrell</name>

src/main/java/org/p2p/solanaj/core/Message.java

@@ -41,7 +41,7 @@ int getLength() {
     private String recentBlockhash;
     private AccountKeysList accountKeys;
     private List<TransactionInstruction> instructions;
-    private Account feePayer;
+    private PublicKey feePayerPublicKey;
 
     public Message() {
         this.accountKeys = new AccountKeysList();
@@ -143,8 +143,8 @@ public byte[] serialize() {
         return out.array();
     }
 
-    protected void setFeePayer(Account feePayer) {
-        this.feePayer = feePayer;
+    protected void setFeePayerPublicKey(PublicKey feePayerPublicKey) {
+        this.feePayerPublicKey = feePayerPublicKey;
     }
 
     private List<AccountMeta> getAccountKeys() {
@@ -159,7 +159,7 @@ private List<AccountMeta> getAccountKeys() {
                     .collect(Collectors.toList());
         }
 
-        int feePayerIndex = findAccountIndex(keysList, feePayer.getPublicKey());
+        int feePayerIndex = findAccountIndex(keysList, feePayerPublicKey);
         List<AccountMeta> newList = new ArrayList<AccountMeta>();
         AccountMeta feePayerMeta = keysList.get(feePayerIndex);
         newList.add(new AccountMeta(feePayerMeta.getPublicKey(), true, true));

src/main/java/org/p2p/solanaj/core/Transaction.java

@@ -5,6 +5,7 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
+import java.util.function.Function;
 
 import org.bitcoinj.core.Base58;
 import org.p2p.solanaj.utils.ShortvecEncoding;
@@ -76,8 +77,7 @@ public void sign(List<Account> signers) {
         }
 
         Account feePayer = signers.get(0);
-        message.setFeePayer(feePayer);
-
+        message.setFeePayerPublicKey(feePayer.getPublicKey());
         serializedMessage = message.serialize();
 
         for (Account signer : signers) {
@@ -91,6 +91,29 @@ public void sign(List<Account> signers) {
         }
     }
 
+    /**
+     * Signs the transaction with external signer.
+     *
+     * @param feePayerPublicKey - The public key of the signer's account.
+     * @param externalSigner - Function for external sign.
+     * @throws IllegalArgumentException if no signers are provided
+     */
+    public void signByExternalSigner(PublicKey feePayerPublicKey, Function<byte[], byte[]> externalSigner) {
+        if (externalSigner == null) {
+            throw new IllegalArgumentException("No external signer provided");
+        }
+
+        message.setFeePayerPublicKey(feePayerPublicKey);
+        serializedMessage = message.serialize();
+
+        try {
+            byte[] signature = externalSigner.apply(message.serialize());
+            signatures.add(Base58.encode(signature));
+        } catch (Exception e) {
+            throw new RuntimeException("Error signing transaction", e); // Improve exception handling
+        }
+    }
+
     /**
      * Serializes the transaction into a byte array.
      *

src/main/java/org/p2p/solanaj/rpc/RpcApi.java

@@ -13,6 +13,7 @@
 import org.p2p.solanaj.ws.listeners.NotificationEventListener;
 
 import java.util.*;
+import java.util.function.Function;
 import java.util.stream.Collectors;
 
 public class RpcApi {
@@ -91,6 +92,37 @@ public String sendTransaction(Transaction transaction, List<Account> signers, St
         return client.call("sendTransaction", params, String.class);
     }
 
+    /**
+     * Sends a transaction to the RPC server with external signer
+     *
+     * @param transaction - The transaction to send.
+     * @param feePayerPublicKey - The public key of the signer's account.
+     * @param externalSigner - Function for external sign.
+     * @param recentBlockHash - The recent block hash. If null, it will be obtained from the RPC server.
+     * @param rpcSendTransactionConfig - The configuration object for sending transactions via RPC.
+     * @return The transaction ID as a string.
+     * @throws RpcException If an error occurs during the RPC call.
+     */
+    public String sendTransaction(Transaction transaction, PublicKey feePayerPublicKey,
+                                  Function<byte[], byte[]> externalSigner, String recentBlockHash,
+                                  RpcSendTransactionConfig rpcSendTransactionConfig) throws RpcException {
+        if (recentBlockHash == null) {
+            recentBlockHash = getLatestBlockhash().getValue().getBlockhash();
+        }
+        transaction.setRecentBlockHash(recentBlockHash);
+        transaction.signByExternalSigner(feePayerPublicKey, externalSigner);
+        byte[] serializedTransaction = transaction.serialize();
+
+        String base64Trx = Base64.getEncoder().encodeToString(serializedTransaction);
+
+        List<Object> params = new ArrayList<>();
+
+        params.add(base64Trx);
+        params.add(rpcSendTransactionConfig);
+
+        return client.call("sendTransaction", params, String.class);
+    }
+
     /**
      * Sends a transaction to the network for processing.
      * A default RpcSendTransactionConfig is used.

src/test/java/org/p2p/solanaj/core/MessageTest.java

@@ -19,7 +19,7 @@ public void serializeMessage() {
         Message message = new Message();
         message.addInstruction(SystemProgram.transfer(fromPublicKey, toPublickKey, lamports));
         message.setRecentBlockHash("Eit7RCyhUixAe2hGBS8oqnw59QK3kgMMjfLME5bm9wRn");
-        message.setFeePayer(signer);
+        message.setFeePayerPublicKey(signer.getPublicKey());
 
         assertArrayEquals(new int[] { 1, 0, 1, 3, 6, 26, 217, 208, 83, 135, 21, 72, 83, 126, 222, 62, 38, 24, 73, 163,
                 223, 183, 253, 2, 250, 188, 117, 178, 35, 200, 228, 106, 219, 133, 61, 12, 235, 122, 188, 208, 216, 117,