Proposal: clarify codec/serialization module boundaries under src/

[darion-yaphet]

src/json2pb and src/mcpack2pb are currently peer directories of src/brpc, although they are primarily payload codec/adapter libraries rather than complete RPC protocol implementations.

This layout has historical and reuse-oriented rationale: both are public, reusable components, and mcpack2pb also backs protoc-gen-mcpack. However, it is not obvious to new contributors why they are outside src/brpc/policy.

Would maintainers be open to either:

1. documenting the ownership and boundaries of top-level src/ modules; or
2. gradually grouping codec-like modules under a clearer namespace/directory, while preserving existing exported include paths?

I understand that a physical move may be source-compatible only with forwarding headers and would also need to account for build rules and protoc-gen-mcpack.

This issue is intended to discuss direction before proposing code changes.

[wwbmmm]

I think there is no need to change the directory structure.