Update GitLab Migration to use source_id when requesting relations export

What does this MR do and why?

We've recently added a new column source_xid to bulk_import_entities table, in order to utilize it's value when making a network request to source GitLab instance to export project/group relations, as part of GitLab Migration feature.

The idea is to make a network request to /api/v4/groups/12345/export_relations instead of /api/v4/groups/group_full_path/export_relations in order to avoid a potential lookup clash, when full path is used, as a user can have a group with all numerical full path (e.g. 5000). In this case, our REST API treats and resolves it as integer id instead of string full_path. In order to avoid this problem, we now store source_xid in the database for future use, instead of relying on full_path.

This MR updates BulkImports::ExportRequestWorker in order to try update the corresponding entity source_xid if it is missing, in order to make a network request with id instead of full path. If that fails, fallback on using full path instead.

Screenshots or screen recordings

source_xid

How to set up and validate locally

1. Feature.enable(:bulk_import)
2. Feature.enable(:bulk_import_projects)
3. Create a group with a project in it
4. Navigate to /groups/new#import-group-pane & enter gdk instance url & password
5. Select a group your created to be migrated under a new destination/name
6. Wait for migration to be complete & verify group & project are still migrated
7. Verify rows in bulk_import_entities table have source_xid set

MR acceptance checklist

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