add feature for file: URL support

there are a number of situations where file: URL support is not desirable:
* server side code, where file: URL could be an attack vector
* WASM code, where file access is not available (and won't even compile)

jsonld/Cargo.toml

@@ -14,7 +14,9 @@ keywords.workspace = true
 
 [features]
 default = []
-# This feature enables the JSON-LD parser and serializer
+# This feature enables retrieval of context via file: URLs
+file_url = []
+# This feature enables retrieval of context over HTTP(S)
 http_client = ["json-ld/reqwest"]
 
 [dependencies]

jsonld/src/loader.rs

@@ -25,7 +25,9 @@ pub type FsLoader =
 mod static_loader;
 pub use static_loader::*;
 
+#[cfg(feature = "file_url")]
 mod file_url_loader;
+#[cfg(feature = "file_url")]
 pub use file_url_loader::*;
 
 #[cfg(feature = "http_client")]

resource/Cargo.toml

@@ -17,7 +17,9 @@ keywords.workspace = true
 jsonld = ["sophia_jsonld", "futures-util"]
 # This feature enables the RDF/XML parser and serializer
 xml = ["sophia_xml"]
-# This feature enables the HTTP client in dependencies
+# NOT IMPLEMENTED YET: This feature enables retrieval of resource graphs over file: URLs
+file_url = []
+# NOT IMPLEMENTED YET: This feature enables retrieval of resource graphs over HTTP(S)
 http_client = []
 
 

sophia/Cargo.toml

@@ -21,6 +21,8 @@ jsonld = ["sophia_jsonld", "sophia_resource/jsonld"]
 xml = ["sophia_xml", "sophia_resource/xml"]
 # This feature enables to use the graph and dataset test macros in other crates
 test_macro = ["sophia_api/test_macro"]
+# This feature enables the file: URL support in dependencies
+file_url = ["sophia_jsonld/file_url", "sophia_resource/file_url"]
 # This feature enables the HTTP client in dependencies
 http_client = ["sophia_jsonld/http_client", "sophia_resource/http_client"]