Traversal ids queries

What does this MR do?

Closes #230456 (closed)

Currently to query the namespace hierarchy we must use recursive methods through Gitlab::ObjectHierarchy. We can query the namespace hierarchy a lot faster and easier if we store the path from the root ancestor to each namespace as an attribute on the namespace. We will store this on a new traversal_id attribute on Namespaces.

Say we have this Namespace hierarchy...

graph TD;
    gitlab-->backend;
    backend-->create;
    backend-->manage;
    create-->source;
    manage-->access;
    gitlab-->frontend;

Then the path from gitlab to access is gitlab / backend / manage / access. The path from gitlab to source is gitlab / backend / create / source.

We can use this structure for fast queries. If the current namespace is backend then all our descendants match the path gitlab / backend / *, all our ancestors match the path * / backend, and everyone in the same Namespace hierarchy matches gitlab / *.

We store that path using an array of Namespace ids in a new column on Namespace called traversal_ids.

The recursive Namespace query methods have been moved to a module at app/models/concerns/namespace/recursive_traversal.rb. Feature flags have been included to toggle between using recursive and linear querying methods.

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