Add basic GitHub action for running maven build and test

[mfoo]

This change adds a GitHub pipeline that runs a Maven build and JUnit tests to ensure that builds are green before code is merged. This should help prevent any contributions accidentally introducing issues.

Notes:

• It runs on all pushes and again on merge request creation (since the target branch may be ahead)
• It uses ubuntu:latest and JDK 17. From getting this running on my Pi 4B on 22.04.1 LTS this has worked just fine, and JDK 16 that is currently being used is no longer supported so it's a good time to encourage moving away from it.
• It doesn't yet report the details of test failures back to GitHub but actions exist to do that - I just haven't tested them and wanted to get something simple merged first.

Having a build/test pipeline is a prerequisite to introducing something like Dependabot to automatically keep dependencies up-to-date - we don't want to accidentally merge dependency bumps that break the build :smile:

Edit:

Oops, forgot to say - you can see evidence that this works here:

• See the :heavy_check_mark: to the right of the most recent commit message
• Click the check mark or go to Actions to see the run itself
• Click on "Build" on that page to see the build output.

[abstractfoundry]

Hi @mfoo thank you so much for putting this together, and for all your other great PRs and issues 😀 🏆 ✨ Looking forward to getting these all merged in! The reason we're still using JDK 16 is that we were getting segmentation faults in the arm32 C2 compiler (super bizarre, right) on more recent versions. Is it just a simple matter of changing java-version to '16' to continue building and testing against JDK 16, until we can be sure the compiler issue has been sorted out?

[mfoo]

Should be, yep!

[abstractfoundry]

Awesome, thanks - I'll make that change after merging.