ngclient: return loaded metadata from Updater._load_targets

[sechkova]

Description of issue or feature request:
During the traversal of the delegations tree, each targets metadata file is first loaded/downloaded and then "visited".
It would be convenient if Updater._load_targets directly returns the loaded metadata to avoid lookup on the next line.

Current behavior:

            # The metadata for 'role_name' must be downloaded/updated before
            # its targets, delegations, and child roles can be inspected.
            self._load_targets(role_name, parent_role)
            role_metadata: Targets = self._trusted_set[role_name].signed

Expected behavior:

role_metadata = self._load_targets(role_name, parent_role)

[jku]

This should go one level deeper. right? TrustedMetadataSet should return the new trusted Metadata on successful calls to all update_*() functions?

This doesn't make the task any more complex but means it could be done in two bits: first add the TrustedMetadataSet changes, then use them in Updater._load_targets()

[ivanayov]

Can I take this, please?