Update bulk_create method to also create metadata records

What does this MR do and why?

In order for us to deduplicate commit metadata from the merge_request_diff_commits table to the new merge_request_commits_metadata table, we need bulk creation logic to also create data on the new table. This applies to new MRDC records. A separate backfill migration will be done along when we start to partition MRDC table as well.

This is a temporary situation. Later on as we progress in implementing this deduplication approach, we will be removing the logic to write commit metadata on MRDC table.

This is behind merge_request_diff_commits_dedup feature flag.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/527231

How to set up and validate locally

1. Enable merge_request_diff_commits_dedup feature flag.
2. Create a MR.
3. Add multiple commits to that MR.
4. See the Commits tab on the MR page and they should still display all commits.
5. Compare the merge_request_diff_commits and merge_request_commits_metadata records of the MR and they should all have the same values for the following columns:
   • commit_author
   • committer
   • authored_date
   • committed_date
   • sha
   • message
   • trailers
6. merge_request_diff_commits records should have appropriate merge_request_commits_metadata_id set.
7. merge_request_commits_metadata records should have appropriate project_id set. This should be the project where the MR is created on.

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