feat(compiler): support C++ gRPC code generation

[BaldDemian]

Why?

Support C++ gRPC code generation.

What does this PR do?

Overview

This PR adds C++ gRPC code generation support.
All unary, server-streaming, client-streaming, and bidirectional RPC types are supported.
They integrate with gRPC C++ synchronous APIs.

Fory-backed codec

Generated gRPC code uses Fory as codec. Given that the codec is relatively small, it is generated directed within each *_service_grpc.h.
On writes, codec moves serialized bytes into a gRPC Slice, avoiding an additional outbound payload copy. On reads, it uses a single ByteBuffer slice when possible and merges fragmented buffers only when necessary.

Error-handling

To make the logic more robust, this PR also adds gRPC-related naming collision detection and escaping.
This PR also considers special characters of C++ such as compilation guards.
Generated model and service headers use sanitized include guards;
the shared Fory codec is protected by FORY_GENERATED_GRPC_SERIALIZATION_TRAITS_;
each grpc::SerializationTraits<T> specialization has a stable, type-specific guard derived from a sanitized fully qualified type name and a SHA-256 prefix. This prevents duplicate specializations when several generated service headers use the same message type.

Testing

This PR adds compiler-side pytests and C++-Java gRPC interop tests. It also adds a dedicated job in the CI.

Documentation

This PR adds documentation for the usage and example of the C++ gRPC code generation feature.

Related issues

Close #3276.

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence or equivalent persisted links of the final clean AI review results from both fresh reviewers described in AI_POLICY.md, the Fory-guided reviewer and the independent general reviewer, on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

N/A.

Benchmark

N/A.