[chore(gRPC)]: Update a2a.proto to include metadata on GetTaskRequest

[fjuma]

See a2aproject/A2A#996

We need to update ProtoUtils accordingly to be able to handle metadata.

[kabir]

This appears to have been removed since the mentioned PR. I don't see it in https://github.com/a2aproject/A2A/blob/main/specification/grpc/a2a.proto.

It was reverted here a2aproject/A2A@e6b8c65

[kabir]

The latest .proto file doesn't contain this. If it is introduced, our periodic updates will include it