Migration observer to report size change

What does this MR do?

This aims to add reporting on the total database size change by a given migration (during migration testing).

We introduce the concept of a "migration observer" here - since we plan on making a whole lot of "observations" for a given migration. So this already aims to make this extensible easily.

Screenshots (strongly suggested)

https://gitlab.com/project_278964_bot2

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