TCN-463 rebuild CI in GitHub actions

[elliotmjackson]

When we move the repo, the CircleCI trigger and connection will likely be severed. Even if it isn't, we do not own the envoy CircleCI instance and cannot manage it moving forward.

Utilising the keys we obtained in TCN-461, we can build a set of Github actions which:

1. Build and test on PR
2. Build, test and publish on merge to main for package managers
   • mvn
   • pypi