Add CI workflow with coverage reports

[Peetee06]

Let's add a Continuous Integration workflow that ensures proper formatting, checks linter rules and runs tests.

We can also include codecov, as it's free for Open Source projects and has really nice visualizations of coverage.

This will allow the repo to become more robust which results in more trust and wider adoption.

If you are open to it, I will contribute the workflow for the Dart/Flutter code.

All I need from a maintainer is to setup the CODECOV_TOKEN via codecov for the msal_auth repository.