Support automatic sumfile updating

[ericcornelissen]

Summary

A major limitation of ghasum is that automated GitHub Actions updates through services like Dependabot and Renovate do not automatically update the ghasum checksums. To make it more convenient to use ghasum this should be automated.

Proposal

One way in which this can be automated is with an action that can be triggered on Pull Requests from update services (e.g. using a branch filter) which basically runs ghasum update and commits the result back to the same branch.