Add an initial config for build/test using Github Actions

[peterebden]

Lots to do still:

• Persist artifacts between jobs
• Set up the release action
• Can we reuse steps using composite actions or similar? Repeating the archive_logs & cache steps is tedious.
• Add rex_test and http_cache_test (needs persisted artifacts)

Main goal here is to see if this works (I'm sure it will need some iteration) and how fast it ends up being. The runners are apparently smaller and I'm not sure we can increase them, which may make a difference for us.