Cells: Put `namespaces` and `users` into dedicated gitlab schemas

What does this MR do and why?

This MR does:

• Add gitlab_main_cell and put namespaces into it
• Make users and put it into gitlab_main_clusterwide
• This temporarily allows all cross-joins, cross-DBs and cross-FKs between those.

To achieve this, it does extend gitlab_schemas/*.yml with:

• allow_cross_joins: that defines all other schemas that are allowed to be used together for cross joins
• allow_cross_transactions: when running transaction {} with distinct queries
• allow_foreign_keys: to hold foreign keys between tables in those schemas

Resolves

• Cells: Iteration 2: Make `users` to be `gitlab_... (#410370 - closed)
• Cells: Iteration 2: Introduce `gitlab_main_cell... (#409839 - closed)
• Cells: Iteration 2: Make `namespaces` to be `gi... (#390102 - closed)

References

• Based on: Cells: Store databases and gitlab schema config... (!120484 - merged)

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.