Add ProjectNamespace policy

What does this MR do and why?

This policy is currently empty -no actions are allowed for any roles because existence of this resource is hidden to users for now. In future we will add incrementally permissions when migrating functionality to ProjectNamespace.

For now the main reason for existence of this policy is to assure that no ProjectNamespace is accidentally created (in case we wouldn't check namespace type on some place).

Related to #340377 (closed)

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