C/C++: add server callbacks for on_un/subscribe

[jtbandes]

Changelog

• C/C++: added server options with callback functions for subscribe and unsubscribe
• C/C++: added getter for channel id

Docs

N/A

Description

In addition to the features mentioned above:

• Updated C++ example server to log subscribe/unsubscribe events
• Added end-to-end C++ test using websocketpp asio client
• Changed some string_view to string because I learned that string_view is not necessarily null-terminated, but on the Rust side we assume it is

[linear]

FG-10676 C/C++: add WS server listener