We want to start importing Group and Project data directly from
another GitLab instance, via the API. To do this, we need somewhere
to store authentication data, entity mapping data and other metadata
so that we can perform the import in background jobs.

Create the BulkImport model which is associated with a user, and used to
group together all the data relating to the one bulk import request from
the user. This will be followed up by the introduction of the other
models the store the import data.

This model introduces:

1. `source_type` -> where the data for this import is being sourced
   from. For now this will just be GitLab imports, but in the future we
   could expand this structure to other services like GitHub and
   Bitbucket.
2. `user_id` -> the user that initiated the import.
3. `status` -> to be used by the state machine for recording the status
   of the overall import.

This is a component of the Group Migration MVC epic:
https://gitlab.com/groups/gitlab-org/-/epics/4374

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42978
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/250280