websocket: transition WebSockets to use SoupMessageHeaders

The Cockpit prject has its own header parsing code because it doesn't
depend on libsoup. Now that it is integrated within libsoup, we can
drop that code.
tests: add websocket tests from the Cockpit project
websocket: expose get_accept_key() method
tests: websocket: switch to libsoup's coding style
websocket-connection: switch away from raw url string to use SoupURI