Extend index on ci_builds_metadata

What does this MR do?

This extends an existing index and adds covering information to it. This is based on the suggestion in #296755 (closed).

Note that add_index doesn't have support for covering indexes. If we need this more often, we can move this into a migration helper. For now, I wouldn't want to bother and wait until we see repetition.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content