GitHub actions CI workflows

[owenv]

• macOS/Linux/Windows tests
• cmake smoke test
• soundness checks

[shahmishal]

Can we move the macOS, Linux and Windows CMake logic into swiftlang/github-workflows?

[owenv]

@shahmishal Right now the cmake-smoke-test relies on a SwiftPM command plugin which only exists in a couple repos and isn't really generalizable. I could maybe move only the CMake install step to the workflows repo, does that makes sense to you?

[shahmishal]

@etcwilde I remember you were planning on upstreaming the changes from CMake example repo into the swiftlang/github-workflows.

@owenv you can merge this for now, and we can clean this up later.