fix(client): align send_message signature with BaseClient (#740)

# Description
Aligned the `send_message` signature in `Client` with `BaseClient`.

Note: I used `SendMessageConfiguration` (instead of
`MessageSendConfiguration` mentioned in the issue) because
`SendMessageConfiguration` is the correct attribute name found in
`a2a_pb2`, as verified by `mypy`.

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Fixes #727 🦕

---------

Signed-off-by: Akshat Kumar <akshat230405@gmail.com>
Co-authored-by: Ivan Shymko <ishymko@google.com>

Author: Akshat8510

src/a2a/client/client.py

@@ -19,6 +19,7 @@
     ListTasksResponse,
     Message,
     PushNotificationConfig,
+    SendMessageConfiguration,
     StreamResponse,
     SubscribeToTaskRequest,
     Task,
@@ -111,6 +112,7 @@ async def send_message(
         self,
         request: Message,
         *,
+        configuration: SendMessageConfiguration | None = None,
         context: ClientCallContext | None = None,
         request_metadata: dict[str, Any] | None = None,
         extensions: list[str] | None = None,