Instrument database migrations

What does this MR do?

This adds a rake task to execute pending database migrations with added instrumentation. This is only relevant in context of "database testing", see !51347 (merged).

The idea is to:

1. Execute rake gitlab:db:migration_testing[somefile.json] on a thin clone
2. This produces observations captured in somefile.json
3. The json file is being used to report those observations back to the MR

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