Add group membership proto (#167)

## tl;dr

Add `GroupMembership` proto in line with [XIP-46](https://community.xmtp.org/t/xip-46-multi-wallet-identity/639)

proto/mls/message_contents/group_membership.proto

@@ -0,0 +1,13 @@
+// Group membership
+syntax = "proto3";
+
+package xmtp.mls.message_contents;
+
+option go_package = "github.com/xmtp/proto/v3/go/mls/message_contents";
+option java_package = "org.xmtp.proto.mls.message.contents";
+
+// Contains a mapping of `inbox_id` -> `sequence_id` for all members of a group.
+// Designed to be stored in the group context extension of the MLS group
+message GroupMembership {
+  map<string, uint64> members = 1;
+}