Cells: Store databases and gitlab schema configuration in YAML

What does this MR do and why?

Describe in detail what your merge request does and why.

Cells: Store databases and gitlab schema configuration in YAML

The purpose of this change is to provide a better and more structured way to define gitlab_schemas and database_connections.

This helps to define applied behavior for connections, and what schemas the connection holds, and how those are configured (load balancing, etc.).

Each DB and schema is named, and contains description.

It should serve as a single source of truth.

Related

• Improve how database configurations and gitlab ... (#410797 - closed)
• Cells: Iteration 2: Introduce `gitlab_main_cell... (#409839 - closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.
• I confirmed that all checkboxes are marked in !120484 (comment 1400102930)