More code sharing

[jku]

I'd like signing-event to output more details about the changes in the signing event. Some of this already exists in tuf-on-ci-sign.

Unfortunately this is where the design decision of two separate code bases (signer and repo) starts to fail a bit: code sharing is tricky. Two options I can see:

• create a library "tuf-on-ci-common". This seems too much: I don't want anyone else using it
• use hatchlings forced inclusion https://hatch.pypa.io/latest/config/build/#forced-inclusion

I think the latter is worth trying: we could have a common/ directory that both repo and signer force-include.