Resolve tags into UMTHES using search API

When we collect stringly-typed tags into Tag::Other values, it might still make sense to try to resolve them into UMTHES using their search API to replace these tags by the Tag::Umthes variant. The main issue here is integration with the existing caching mechanisms, e.g. we might want to have a separate cache for the search API responses (persistent or in-memory) and then tie this in with the existing tag definition storage.