Add new gitlab_main_cluster_setting gitlab schema

What does this MR do and why?

Add new gitlab_main_cluster_setting gitlab schema

This new schema is meant to be completely isolated from other schemas for FKs.

This is the 1st part of many MRs to implement https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/decisions/014_clusterwide_syncing_in_cells_1_0/#decision.

Also we reduce number of gitlab_main_clusterwide tables.

References

• #521079 (closed)
• https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/decisions/014_clusterwide_syncing_in_cells_1_0/#decision
• #515628 (closed)

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