Add contentHint and groupId fields to UnidentifiedSenderMessageContent

And to the ProtocolExceptions for Java, thrown when a sealed sender
message's content fails to decrypt. (Eventually all languages will
support this.)

rust/bridge/ffi/src/lib.rs

@@ -140,6 +140,6 @@ pub unsafe extern "C" fn signal_sealed_session_cipher_decrypt(
         write_optional_cstr_to(sender_e164, Ok(decrypted.sender_e164))?;
         write_cstr_to(sender_uuid, Ok(decrypted.sender_uuid))?;
         write_result_to(sender_device_id, decrypted.device_id)?;
-        write_bytearray_to(out, out_len, decrypted.message)
+        write_bytearray_to(out, out_len, Some(decrypted.message))
     })
 }