Add PolicyCheckable concern for things passing to policy check

What does this MR do?

Add PolicyCheckable concern for things passing to policy check

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #7000 (closed)