Add frontend action and backend route to reset integration

What does this MR do?

Related to #273778 (closed)

This MR is a follow-up of !47188 (merged). In this MR, we add the Vuex action and make the UI fully functional but hidden behind a feature flag. I also add the basic backend code with routes and controller action.

There shouldn't be any problems merging this as-is since we have a development feature flag reset_integrations used to hide the frontend button and modal.

Screenshots (strongly suggested)

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