Add `gitlab:db:validate_config` to ensure the proper `database_tasks:` is set

What does this MR do and why?

Resolves part of: #356009 (closed).

This implements gitlab:db:validate_config to ensure correctness of used database_tasks::

• A database configuration using database_tasks: true should not share this database with other jobs,
• A database configuration using database_tasks: false should share the database with main:

This is done by doing a simple dependency rake task chained to db:migrate/db:schema:load/dump that:

• Connects to each database configuration
• Gets unique system identifier and current connected database
• Performs a set of validation checks according to above criteria

The purpose of this check is to prevent running db:migrate/db:schema:load (and potentially loosing data) if the configuration is invalid. This rake task will raise an exception in such case.

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