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 a mapping between the source entity and the destination
entity.

Create the Entity model which is associated with a BulkImport,
and used to store the data required to link the entity (project or
group) on the source instance and the destination instance.

This model introduces:

1. `bulk_import_id` -> the bulk import that this data belongs to.
2. `parent_id` -> the parent ImportEntity that this one should be
   imported into
3. `source_type` -> what kind of entity this is (group or project)
4. `source_full_path` -> path to access the entity on the source
5. `destination_name` -> what to call the entity on the when it's
   created
6. `destination_namespace` -> where to store the entity on the
   destination.
7. `status` -> for use with the state machine to determine the import
   status of this entity
8. `jid` -> the job ID of the sidekiq job that will/has processed this
   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