* feat(http2): add http2_request structure for server-side request handling

Add http2_request struct to represent incoming HTTP/2 requests with:
- Pseudo-headers parsing (:method, :path, :scheme, :authority)
- Regular headers storage
- Helper methods for content-type, content-length
- Request body handling from DATA frames
- Validation for RFC 7540 compliance

Part of #406, closes #433

* feat(http2): add http2_server_stream class for response handling

Implement http2_server_stream class providing server-side stream handling:
- Response headers with status code via HPACK encoding
- Data frame sending with automatic frame splitting
- Streaming response support (start_response, write, end_response)
- Stream reset functionality
- Flow control window management
- Thread-safe state management

Part of #406, closes #434

* feat(http2): implement http2_server class for HTTP/2 server support

Add HTTP/2 server implementation with:
- http2_server: Main server class with TLS and non-TLS support
- http2_server_connection: Per-connection handler with frame processing
- Connection acceptance with ALPN "h2" negotiation
- SETTINGS exchange per RFC 7540
- Stream multiplexing support
- Flow control (connection and stream level)
- HPACK header compression/decompression
- Request handler callback API
- Cleanup timer for dead connections
- Proper GOAWAY handling on shutdown

Also updates CMakeLists.txt to include new source files.

Part of #406, closes #435

* Add unit tests for HTTP/2 server implementation

Add comprehensive tests for:
- http2_request: header parsing, content helpers, validation, from_headers
- http2_server: construction, settings, lifecycle, handlers
- http2_settings: default values
- tls_config: default values and property setting

All 28 tests passing.

Refs #406
Closes #436

* Add HTTP/2 server example demonstrating basic usage

Example demonstrates:
- Server creation and configuration
- HTTP/2 settings customization
- Request handler with routing
- Various endpoints (health check, echo, POST data)
- Graceful shutdown handling

The example uses h2c (HTTP/2 cleartext) for simplicity.
Production usage should use start_tls() with proper certificates.

Refs #406
Closes #437

* Update documentation for HTTP/2 server implementation

- Add HTTP/2 server implementation entry to CHANGELOG.md
- Update FEATURES.md roadmap: HTTP/2 now complete
- Update planned enhancements to reference http2_server class

Refs #406