Optimize AvailableLabelsService for multiple labels

What does this MR do?

PUT /api/:version/groups/:id/(-/)epics/:epic_iid executes almost 100 queries, in case of failing test particulart number is 118

This is a subset of changes required to optimize this endpoint. The change made in this MR is optimizing AvailableLabelsService and Labels::FindOrCreateService to not query multiple times labels by passing label titles to the scope.

Before:

PUT /api/:version/groups/:id/(-/)epics/:epic_iid(.:format): a maximum of 110 is allowed but 118 SQL queries were executed

After:

PUT /api/:version/groups/:id/(-/)epics/:epic_iid(.:format): a maximum of 110 is allowed but 113 SQL queries were executed

From 118 SQL to 113. -5 queries for example with 6 labels in params. With more labels, we'll get more impressive results.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #194104 (closed)