Add ability to add an "Approval Gate" approval rule to a Project (BE)

Why are we doing this work

As a first step towards implementing External API Approval Rules for compliance checks, we need to be able to add an external "Approval Gate" to the approval rules for a project.

Relevant links

• Epic
• Design

Implementation plan (backend only)

• Create feature flag for epic like compliance_approval_gates.
• Create new type of ApprovalProjectRule, probably extending the ApprovalRuleLike concern.
• Extend REST API to allow new type of rule to be created.

Out of scope

This issue does not cover anything other than the ability to add an Approval Gate to a project. Specifically, external HTTP calls and callbacks. This issue also does not cover any frontend aspects, which will be handled under #300973 (closed).