[API] Approval rule endpoints

What does this MR do?

Add API endpoints for frontend to manipulate approval rules.

The list of APIs are here: https://docs.google.com/spreadsheets/d/1LoosFEFsWPmZhB5f6MnGq0fxp1zCoqyGOOG0uFmd2MA/edit?usp=sharing The entity descriptions can be found here: #8350 (closed)

What are the relevant issue numbers?

#8789 (closed)

Frontend #8350 (closed)

Source issue #1979 (closed)

Recommended readings

This feature is part of #1979 (closed), implementing multiple approval rules.

The schema for new tables can be seen here https://gitlab.com/gitlab-org/gitlab-ee/issues/8647

The core business logic (e.g. calculating approved?, approvers_left) on the model side are mainly located at ApprovalState and ApprovalWrappedRule. Those are wrappers around the AR models, see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8974

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer