docs: Add gRPC Post-Quantum Cryptography Guide

[lqiu96]

Summary

This pull request expands the Post-Quantum Cryptography (PQC) User Guide in docs/post_quantum_cryptography_guide.md to cover gRPC transport in addition to HTTP/JSON.

This PR is stacked on top of PR #13963 (pqc-user-guide).

Highlights

• General PQC Architecture: Updates the guide to introduce PQC concepts, the Store-Now, Decrypt-Later (SNDL) threat model, and hybrid key exchange across Google Cloud Java client libraries.
• Dedicated gRPC Transport Section (Section 3):
  • Details the default grpc-netty-shaded transport engine bundling BoringSSL (netty-tcnative-boringssl-static) with built-in PQC hybrid key exchange support (X25519MLKEM768) in gRPC-Java 1.83.0+.
  • Explains handshake negotiation with Google Cloud Front End (GFE) endpoints and graceful fallback to classical TLS.
  • Explains gRPC HTTP/2 long-lived multiplexed channels (ManagedChannel) and how TLS overhead occurs once upon connection establishment.
• Verification (Section 7):
  • Explains why standard JVM -Djavax.net.debug flags do not capture gRPC handshakes when using grpc-netty-shaded.
  • Documents Netty SSL debug logging (-Dio.grpc.netty.shaded.io.netty.handler.ssl=DEBUG) and ClientInterceptor response header verification.
• Custom Configurations (Section 8):
  • Documents custom Netty SSL context configuration via InstantiatingGrpcChannelProvider.Builder.setChannelConfigurator(...).

[gemini-code-assist]

Code Review

This pull request expands the Post-Quantum Cryptography (PQC) User Guide to cover both gRPC and HTTP/JSON transports, adding comprehensive sections on gRPC transport architecture, platform compatibility, verification methods, and custom configurations. The review feedback suggests correcting the Netty SSL logging instructions to configure the logging framework directly rather than using a JVM system property, and removing an unused import in the gRPC configuration code snippet.