HTTP header inclusion

[BenTheCyberOne]

Summary

This PR adds support for sending custom HTTP headers during the WebSocket (wss://) handshake in the Ligolo-ng agent.

A new CLI flag -header has been introduced, which can be repeated multiple times to specify one or more headers in Key: Value format.

Example usage:

./agent -connect https://proxy.example.com:443 \
        -header "X-Test: 123" \
        -header "X-Trace: abc"

[DrorDvash]

handy! thank you