Add namespace_features table

What does this MR do?

Add namespace_features table, similar to project_features table.

Part of #208412 (closed)

Based on gitlab-foss!15060 (closed) by @randaalex

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.