Enable Refresh() to be called more than once

[rdimitrov]

Moved from rdimitrov/go-tuf-metadata#86

Describe the bug

Currently a Refresh() can be done only once during the lifetime of an Updater.

This is not optimal for long-living processes so it would be better if we enable calling Refresh() more than once.

References:

• https://github.com/rdimitrov/go-tuf-metadata/blob/4e20edf17ab0b724127ca3d2374e5a9580f3b519/metadata/updater/updater.go#L98C1-L98C72

Expected behavior

Upon calling Refresh() make sure everything is up-to-date and if that's true:

• return nil, instead of an error (better imho)
  or
• return a custom error type so the user can check it via errors.Is (less favourite)

If something else occurred which failed us to refresh the metadata, still return the appropriate error value.

Additional context

No response