Implement mutual TLS (mTLS) authentication for the gateway

[bburda]

Summary

Implement mutual TLS (mTLS) authentication for the gateway to enable certificate-based client authentication, required for secure vehicle diagnostics scenarios.

The codebase has 9 TODO markers for mTLS support across configuration, server setup, and launch files. This is a planned security feature for production deployments.

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Proposed solution (optional)

Configuration Parameters

• ssl_client_cert_path - Path to CA certificate for client verification
• ssl_verify_client - Enable/disable client certificate verification

Implementation Areas

• tls_config.hpp - Add client_ca_cert_path and verify_client fields
• rest_server.cpp - Configure SSL context for client verification
• gateway.launch.py - Add launch arguments for mTLS parameters
• gateway_https.launch.py - Add mTLS configuration

Files to Modify

• src/ros2_medkit_gateway/config/gateway_params.yaml
• src/ros2_medkit_gateway/include/.../tls_config.hpp
• src/ros2_medkit_gateway/src/http/rest_server.cpp
• src/ros2_medkit_gateway/launch/gateway.launch.py
• src/ros2_medkit_gateway/launch/gateway_https.launch.py

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Additional context (optional)

• Current TODOs: Search for TODO.*mutual or TODO.*mTLS in codebase
• cpp-httplib SSL documentation