Draft: Move ExternalPullRequest model to the Ci namespace

What does this MR do?

Move ExternalPullRequest model to the Ci namespace as part of:

#273605 (closed)

ExternalPullRequest becomes Ci::ExternalPullRequest

Screenshots (strongly suggested)

TODO: Verify and add mirroring screenshot

TODO: Verify and add import screenshot

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 this is backstage/feature maintance and should have no effect on the user.
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

No new testing because this is just namespacing.

• 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