Add Namespace#state and Namespace::Detail#state_metadata

What does this MR do and why?

This merge request adds a new "state" feature to namespaces in the GitLab database.

It creates two database migrations that work together:

1. It adds a basic state column to track the current status of namespaces
2. It adds a more detailed state_metadata field that can store additional information about the state as structured data

The changes also include safety measures to ensure state_metadata is stored in the correct format.

This enhancement will allow GitLab to better track and manage different states that namespaces can be in, such as active, archived, or deletion_scheduled. See https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/group_and_project_operations_and_state_management/ for more information.

References

• Related to Add state column to namespaces (#545967 - closed) • Rémy Coutable, Abdul Wadood • 18.5

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