[FEATURE] Add support for gRPC

[edenreich]

Summary

Google's A2A project has a proto file, it should be possible to simply generate a go client and perhaps enhance it with the current ADK functionality.

Acceptance Criterias

• Proto file is added to schemas repo and downloaded to this repo
• A gRPC client is generated from the proto file
• A gRPC server is generated from the proto file
• The default gRPC port is exposed on the container
• gRPC server can be toggled off or on in the configurations, default to false
• Existing functionality works also with gRPC
• It's documented
• Clients connection are explicit - either JSON RPC or gRPC - no need to worry about connection upgrades features etc, out of scope