Improve libwebsockets integration in build

[harveynw]

Using FetchContent at the moment in CMake to get it, even if not using websockets. Could do with a Findlws.cmake type file that is conditional on needing it.

It also requires OpenSSL at the moment to build even though WSS is not used, so that can be disabled.

[joeyshuttleworth]

OpenSSL is/will always be required for master sever operations