Add external approvals to project approvals store

What does this MR do?

This MR updates the Project settings merge request approvals store. This is part 1 of the MRs for #300973 (closed):

1. Add external approvals to project approvals store <- You are here 👈
2. Add external approvals rules to project settings !57528 (merged)

The changes are either unused, or in the case of the fetchRules action hidden behind a development feature flag ff_compliance_approval_gates.

Screenshots (strongly suggested)

No visual changes.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it's behind a development feature flag.
• [-] 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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #300973 (closed)