WIP: Add root_ancestor to namespaces

What does this MR do?

This is a first MR to resolve #218703 (closed). It introduces namespaces.root_ancestor_id to allow preloading to resolve the N+1 issue #218703 (closed)

Database changes

Approximate children namespaces count ~ 468,552

  Index Scan using index_namespaces_on_parent_id_and_id on public.namespaces  (cost=0.56..286415.36 rows=467018 width=329) (actual time=16.541..191346.899 rows=468552 loops=1)
   Index Cond: (namespaces.parent_id IS NOT NULL)
   Buffers: shared hit=308787 read=139401 dirtied=8052
   I/O Timings: read=185566.998

Approximate root namespaces count ~ 7,169,571

 Index Scan using index_namespaces_on_parent_id_and_id on public.namespaces  (cost=0.56..495437.75 rows=7166907 width=329) (actual time=1.416..74392.999 rows=7169571 loops=1)
   Index Cond: (namespaces.parent_id IS NULL)
   Buffers: shared hit=3718538 read=108800 dirtied=6578
   I/O Timings: read=60556.682

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #218703 (closed)