sdk: Forward custom headers to tunnel requests and relays

[Connor Peet (connor4312)]

Summary

• Add static and per-request header providers to the Rust management client, forwarding their values to management HTTP requests and relay WebSocket handshakes.
• Forward TypeScript management-client headers to Node.js relay WebSocket connections, with tests for management request IDs and relay propagation.
• Document that browser WebSocket APIs cannot attach custom handshake headers; browser management requests continue to support them.

This enables VS Code to send its tunnel session and operation IDs and a new request ID for each service request. Consumer dependency updates will follow the SDK release.

Validation

• cargo test --manifest-path rs/Cargo.toml --no-default-features --features connections (20 passed)
• TypeScript tsc --build ts/tsconfig.json
• Focused Mocha suites for additional management headers, per-request IDs, and relay forwarding (3 passed)