Move merge_request_diff_commits commit messages and trailers to object storage

In #331523 (comment 583654940) we found that a lot of storage in this table is used for storing commit messages and trailers. This data is never queried in a structured way, making it an excellent candidate to move to object storage. Combined with #331823 (closed) this would allow us to drastically reduce the size of this table.

The migration process would be more or less as follows:

1. First, add a column to store whatever identifiers/paths/etc we need to determine where in object storage the data is stored. We'd do this for both commit messages and trailers
2. We also need to add two columns to track the migrated state: message_migrated and trailers_migrated. Both are booleans that default to false
3. Add support to the code for reading from object storage if the data is present, falling back to the database data if nothing is found in object storage
4. Add a background migration (scheduled using a post-deployment migration) that starts migrating these rows to object storage. When the data is migrated, the appropriate X_migrated column is set to true
5. In a follow-up release we migrate any remaining data (= rows where migrated is false) in-place. Depending on how fast the migration process takes, this may require more than just one release
6. In this same release we drop the old columns as well as the migrated columns, and adjust the code to start reading from the new data only.