Add CRUD for Instance-Level Integrations

What does this MR do?

Issue #208914 (closed)

In !25752 (merged) we are adding a new section for instance-level integrations. This MR adds the UI and functionality needed to edit / update / test an integration. For context, "integration" is a rename of the "service" model which will be eventually deprecated.

I have not used a feature flag for this since it is not linked in any of the UIs (for now) and not discoverable unless you specifically go to the URL (for example http://localhost:3000/admin/application_settings/integrations/jira/edit).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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