Add import/namespaces controller

What does this MR do?

This MR adds 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: #233059 (closed) #222889 (closed) #27370 (closed)

Removing namespaces logic from base_controller will be done later, when next MRs (switch frontend to using new endpoint) will be merged

Screenshots

N/A

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