Add CI

[olejnjak]

We currently have no CI, at least we should support those scenarios:

• run tests on PRs and main branch pushes
• when #4 is implemented it should also push podspec to Cocoapods trunk repo when new tag is pushed
• probably add a stale bot to close stale issues like we have in our other repos (e.g. Reqres)
• any other case that you consider legit 🙂