Allow pre-populating a ReleaseMetadata instance

By calling .last, the metadata for the latest release-tools tag will be fetched and then pre-populated in a new ReleaseMetadata instance.

All behavior is behind a prepopulate_metadata feature flag.

Additional logging has been added to add_release so that we have visibility into if and when this pre-populated metadata is overwritten.

See gitlab-com/gl-infra/delivery#1646 (closed) and gitlab-com/gl-infra/delivery#1763 (closed).