New API endpoint: Fetch single tag

In gitlab!141838 (merged), we introduced the usage of the list repository tags API which accepts a parameter name that it uses to filter tags.

We have received an issue where multiple tags match the name so when getting .first, a different tag can be returned. Because of this, we added the filtering for the exact match in the rails backend in gitlab!147590 (merged) as a temporary solution.

This issue is to add an endpoint where we can send in the name of the tag and fetch the exact match.