Add namespace_storage_forks_cost_factor to application_settings Table

What does this MR do and why?

Add a namespace_storage_forks_cost_factor column to the application_settings table.

Add a check constraint to ensure the value must be between 0 and 1.

This column will hold the cost factor for forks for namespace storage limits.

• See the initial proposal for cost factors for forks for a general overview of the feature: https://gitlab.com/gitlab-org/gitlab/-/issues/373914
• See this epic for more information: https://gitlab.com/groups/gitlab-org/-/epics/10100

A PoC for this column demonstrating how it will be used is here: !116328 (closed)

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/404628

MR acceptance checklist

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