infra: add CI for tests and release

[Alex]

Now, that the first 🐛 is found and fixed with #16 that brought in some tests it would be good to add a CI.

The idea is to try Github Actions for:

• a test profile: run go test .
• a release profile: build the CLI report generation binary (macOS, linux), create GH release and upload binary in there

An (upcoming #15) server binary and CD for it is going to be handled in a separate issue

[Alex]

We could have more end-to-end tests, as #37 introduced a -test mode to the server, so we could verify things with simple automation like curl -s "http://localhost:8080/?json" | jq -c . | less -S | wc -l