This MR adds a new, tiny `import/namespaces` controller.

This controller lists available namespaces for import. Moving this to
separate controller has following rationale:

* `namespaces` are required for each controller, provided in `import/`
  folder. They are identical, does not have any customization, etc.

* ATM `namespaces` are returned as the part of `status` in
  `base_controller`. With our intention to add pagination to `status` we
  need to move "static part of the content" (namespaces) away from part,
  which will be paginated (repos)

Related issues:

* https://gitlab.com/gitlab-org/gitlab/-/issues/233059
* https://gitlab.com/gitlab-org/gitlab/-/issues/222889
* https://gitlab.com/gitlab-org/gitlab/-/issues/27370