added webhook verification to mux_ruby + helper-func infrastructure

[eropple]

• added webhook verification to Ruby SDK
• added unit tests for happy and sad paths
• deleted all the old stub unit tests
• unit tests (real ones) now run in CI
• updated build system to copy lib-manual to lib on build (has to be done this way because OAS won't remove old files for renamed/deleted modules, so we have to delete lib every build)

[dylanjha]

Just dropping in here to note that we have wanted this for so long. Thanks @eropple for coming up with a pattern to pull in ad-hoc helper functionality into generated SDKs ❤️

[eropple]

Thanks, @dylanjha . I got more of these to do. We'll get there. 😎