Draft: Cells: Add `gitlab_main_cell` and put `namespaces` into it

What does this MR do and why?

This temporarily allows all cross-joins, cross-DBs and cross-FKs between those.

This is subject to be removed once we are able to annotate those tables.

Related

• Based on: Cells: Put `namespaces` and `users` into dedica... (!120605 - merged)
• Based on: Cells: Store databases and gitlab schema config... (!120484 - merged)
• Resolves: Cells: Iteration 2: Introduce `gitlab_main_cell... (#409839 - closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.