Add expiry date to signing event message (when necessary)

[jku]

Extracting this from #68:

• signing event comment could include the current expiry date
• this should be part of tuf-on-ci-status output (the role specific output)
• this output should only happen when the date is relevant (in practice I think if the role is in signing period)
• we should be careful not to use the signing period and expiry date from the branch: we specifically want to look at the values in main

Implementation notes:

• CIRepository._get_signing_status() could figure out the expiry date and add it to a new field in SigningStatus which could then be printed out in _role_status() in signing_event.py
• there is already a CIRepository.signing_expiry_period() to get the periods... but we'll want to be very careful to use either the "old" (main branch) values or "new" (signing event) values in different cases: this might be a good place for some refactoring