Add BulkImports::ExportBatch & BulkImports::BatchTracker models

What does this MR do and why?

This MR adds BulkImports::ExportBatch & BulkImports::BatchTracker models to be used in GitLab Direct Transfer to export/import project/group relations in batches. They're currently not used anywhere and planned for usages in the future MR(s).

Main purpose:

1. ExportBatch - store a batch of 1000 rows for a particular relation export (e.g. first 1000 merge requests of a project)
2. BatchTracker - keep track of state and perform import of a batch of relations, e.g. exported 1000 merge requests mentioned above

Mentions #391221 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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