Implementation of Offline mode for TUF

Sigstore's python client would like to use the TUF updater in a fully offline manner to allow for opt-in offline verification. To do so would require TUF to function using only locally-stored materials.

Within TUF's current implementation, regardless of the validity of already-stored materials, online access is always required. Thus far, we've been able to run the updater on a modified branch using only local materials and seek to expand this modification to allow for opt-in usage of expired metadata in offline mode. This would be useful for clients like sigstore that would to allow for verification with metadata that was previously valid. A primary use case for this would involve verifying on machines that do not necessarily maintain a connection online.

Changes on our experimental branch have been within updater.py and have consisted of adding a new boolean to UpdaterConfig and avoiding online refreshes if said boolean has been set. Further implementation would require changes to trusted_metadata_set.py to skip checks against the expiry of the metadata if the aforementioned boolean has been set.

Ideally the flow of TUF's updater with these changes would go as follows, given the boolean is set:

• Warn users about risks of using offline verification
• Check to see if local metadata exists; exiting with an error message if none is found
• Load local metadata, skipping checks against expiry within trusted_metadata_set.py

Start in updater.py with UpdaterConfig and trace the updater flow for the proposed offline boolean. Then read trusted_metadata_set.py to identify the metadata expiry checks involved. Done means offline mode warns users, errors when local metadata is absent, loads local materials without expiry checks, and avoids online refreshes.