Copy LFS objects when a repository is pull mirrored

Problem to solve

Projects can be mirrored from or to another repository. When activated, we mirror branches, tags, and commits. However, we do not mirror the LFS objects if the repository uses LFS. This is documented in a few places but is certainly not an expected or obvious behaviour and makes mirroring unusable for repositories containing LFS objects.

We should add support for pull mirroring LFS objects.

Intended users

Further details

Proposal

Mirror LFS objects for pull mirroring

Documentation

Update documentation explaining to explain that LFS object mirroring is now supported for pull mirroring.

What does success look like, and how can we measure that?

This is removing a technical limitation of an existing feature, and is not expected to significantly increase usage of mirroring. GitLab's ultimate strategy is remove the need for Git LFS and implement native support for large files. However, in the mean time it is important that we support our existing customers who use LFS and mirroring.