Support v1 Operator Agent synchronous API over gRPC.

Supporting v1 Operator Agent API for synchronous calls over gRPC will enable people to query the agent with more language bindings, and the UI can be based on gRPC. The main work includes:
1. Define the Agent gRPC service in service.proto. The proto will be in proto3, but the request and response messages will still be in proto2, since this is what the current v1 API based on.
2. Implement handlers for gRPC calls to delegate them to the original agent call handlers.