FED-4060 Set up gha-dart-oss

[sydneyjodon-wk]

We should set up gha-dart-oss so we can:

• take advantage of shared workflows
• use the publish workflow to auto-publish on merge 🎉

QA instructions

• CI passes
• All steps previously run in CI are still running even if some of them are in the shared jobs now
• When this merges, follow along to make sure the package actually publishes

[sydneyjodon-wk]

Because I changed this from "build" to "test" - I'll update Rosie to check for the new-named CI checks before this merges

[sydneyjodon-wk]

We can't use the shared unit-test from gha-dart-oss yet because we can't run other commands beforehand - see slack thread