DPL Websocket: Add protocol param to encode_websocket_handshake_reply which allows a response Sec-WebSocket-Accept in handshake.

[nicolaspoffley]

Add protocol param to encode_websocket_handshake_reply which allows a response Sec-WebSocket-Accept in handshake. If no protocol is given to the function, Sec-WebSocket-Accept is not returned in the response.

[github-actions]

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[nicolaspoffley]

@ktf

[alibuild]

Error while checking build/O2/fullCI_slc9 for 00c55fd at 2025-09-24 12:23:

## sw/BUILD/O2-latest/log
/sw/SOURCES/O2/14687-slc9_x86-64/0/Framework/Core/src/HTTPParser.cxx:226:27: error: 'protocol' was not declared in this scope
ninja: build stopped: subcommand failed.

Full log here.