refactor: Separate protobuf dependencies into sensible extras

[vinoo999]

Description

This introduces a proto extra into the package. The core library itself has no direct dependency on protobuf and only rest server and clients require a2a. google-api-core is only a dependency for grpc. Refactor accordingly to reduce dependency mangling introduced by proto for non grpc/rest servers. Assuming a CI regenerates the uv.lock file but can regenerate it otherwise.

This does not follow the runtime validation pattern in #217 as proto dependencies are localized in proto_utils.py.

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