feat: Add configuration verification for targets config

[asraa]

Description

Not for docs: Another thing that might be useful to verify with a verification script, that usage and status are set for all targets

[haydentherapper]

@kommendorkapten I think this is still relevant, could we use trtool to verify the trusted root file?

[kommendorkapten]

Yes, that is correct @haydentherapper

[jku]

I'm not sure what the description means but if this is about validating changes to artifacts like trusted_root.json:

• We should add a separate artifact validation workflow that runs on artifact changes to sign/* branches -- this would not be part of tuf-on-ci but a root-signing workflow that runs in every signing event
• the workflow should run at least one sigstore client with the trusted_root.json (without using TUF)
• the workflow could also run some external validation tooling like trtool