Manage the Actions integration with the CLI

[ericcornelissen]

Relates to #229

Summary

The README.md provides examples of how to integrate ghasum into your CI. It would be nice if the ghasum CLI could help with this. In particular:

• Create the local action (should be fairly easy)
• Update the ghasum version in the local action (should be doable)
• Add the ghasum step to jobs (harder...)