ngclient feature: Add option to only update metadata if needed

This is not full thought out but I'm filing so it's not forgotten.

https://docs.google.com/document/d/1QWBvpwYxOy9njAmd8vpizNQpPti9rd5ugVhji0r3T4c

Sigstore client wants to use TUF to download files but wants to avoid the two mandatory requests (root N+1, timestamp) on startup if it's not necessary. This seems to be a totally valid feature request: if this is feasible we could offer that as an option.

The WIP sigstore client checks the timestamp expiry before creating an Updater: if timestamp is not expired, the client then decides update is not needed. There could be some issues with this:

1. timestamp metadata validity is not checked
2. cached target file validity is not checked
3. root expiry is not checked

these may be partly theoretical worries but it still feels like a hack...

Maybe it is possible to

• offer a (non-spec-compliant) Updater option that verifies
  • that metadata is valid
  • that targets are verified by the metadata
  • without hitting the network if possible
• while still keeping updater implementation readable

?

Start with the linked design document and the ngclient Updater entry point. Determine how an optional metadata-only update path could validate metadata, target verification, and root expiry without network requests when possible. Done means the behavior and non-spec-compliant tradeoffs are clearly defined and covered by appropriate tests.