Add support for exporting repository type data for LFS objects

What does this MR do?

A project can have the same LfsObject linked with up to three LfsObjectsProject records. Each of these records would be for a different repository, recorded in the repository_type property.

The different repositories at time of writing are "project", "design", and "wiki" shortly. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13894 which introduced the recording of this data.

This change exports the list of repository_types as a JSON mapping of oid => repository_types. When imported these let us recreate the correct LfsObjectsProject records.

https://gitlab.com/gitlab-org/gitlab-ee/issues/11090

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers