Break workflow definition into small pieces by using composite actions

[rtyley]

The current reusable-release.yml workflow file is a single, quite large file, which makes getting an overview of what's going on quite hard because the details of each jobs implementation are interleaved with the connective tissue of IO between each job.

We could take those implementations and put them into (composite?) actions.

https://docs.github.com/en/actions/creating-actions/creating-a-composite-action#creating-an-action-metadata-file

github.action_path