feat(mcp): implement MCP integration connectors

[pane2004]

Implements MCP server integration for Flare AI Kit, enabling agents to connect to external tools and services via MCP within TEE environments.

• Add MCPSettings and MCPServerConfig for flexible server configuration via MCP__SERVERS environment variable
  • Servers are not manually added through github.py, etc, and instead dynamic in a Cursor/IDE json format
• Add MCPManager that creates and manages MCP toolsets with support for three transport types:
  • stdio: Local process-based servers (e.g., filesystem, git)
  • SSE: Remote servers using Server-Sent Events
  • HTTP: Remote servers using streamable HTTP
• Integrate with Google ADK's McpToolset for seamless agent tool composition
• Support tool filtering, server enable/disable, timeout configuration, and authentication headers
• Add comprehensive unit tests and documentation

Test plan

• Run uv run pytest tests/unit/mcp/ to verify all unit tests pass
• Configure an MCP server via MCP__SERVERS env var and verify toolset creation.
  Example config for OpenSea:
  MCP__SERVERS='{"OpenSea": {"url": "https://mcp.opensea.io/mcp","headers":{"Authorization": "Bearer <token>"}}}'

Closes #5