Sumfile version 2

[ericcornelissen]

Summary

This is a placeholder issue for creating a second version of the gha.sum file format. The format already supports specifying the version in the file headers (what appears before the first empty line).

Ideas

• Reduce file size by not repeating common prefixes (e.g. group all actions/ actions).
• Provide a ghasum upgrade command that upgrades the gha.sum version file for a project to the latest available version.

[ericcornelissen]

Suggestion by @monperrus:

Split the content of gha.sum by workflow so that it's clearer why each entry exists.

One problem with that approach is that it will likely duplicate some of the checksums. So, I'd prefer some alternative approach that provides the same insight (i.e. which workflow files use which action).

See also #235