_Delegator.verify_delegate provides a binary answer for whether a delegated role has enough valid signatures to meet the threshold of keys defined by its delegator or not.

While this is sufficient for client validation, TUF applications might also be interested in more granular information, e.g. when implementing a distributed signing feature for TUF metadata, so they can tell users how many signatures by what keys are still missing.

Interesting information would be:

• set of valid signatures
• set of invalid signatures (this would require validating all signatures; verify_threshold only verifies until threshold is met)
• optional: eligible keys, used keys, remaining keys (maybe out of scope)

Given that threshold verification is such a crucial part of the Metadata API and can be done wrong so easily, adding a new public function -- e.g. _Delegator.get_delegate_signing_status() -- to Metadata API seems like a good idea.