Extracts the verified mTLS peer certificate (CN, SANs, SHA-256
fingerprint) from the TLS connection in the HTTP handler after the
handshake completes, stores it in the request context, and forwards
it to both gRPC and HTTP auth plugins via new AuthenticateRequest
fields (client_cn, client_san, client_cert_fingerprint).
Remove unused WithContext interface (zero references in codebase), add
doc comments for all 14 exported symbols, and simplify HashFromContext
to match the one-liner pattern used by the other four getters.
Add 22 unit tests covering round-trip, empty context, wrong type, nil
values, multiple values, independent keys, and the Context interface.