Add loose foreign key for ci_job_token_project_scope_links.add_by_id

What does this MR do and why?

Per #348268 (closed) we need to remove this foreign key and replace it with a "loose foreign key" https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html .

It is safest to first add the LFK (as in this MR) and then remove the foreign key in a later MR. In the meantime the LFK will have nothing to cleanup as the foreign key is still cleaning up everything on delete.

Loose foreign keys are cleaned up async so we need to be sure it is safe to remove the foreign key first.

According to #348268 (closed) this is safe because:

No way for user to access once parent is deleted. Please explain: The added_by is used only for tracking purpose and it's not currently displayed

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Related to #348268 (closed)