Resolve alert when end_time is given

What does this MR do?

This resolves an existing Alert when we receive another call to the endpoint with the end_time set. We check if the new payload matches a fingerprint of an existing alert, and if so we resolve the alert.

If no fingerprint matches, then we ignore the occurrence of that alert.

This matches the behaviour for what we do with Prometheus Alerts, in ProcessPrometheusAlertService.

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

Closes #13402 (closed) and #217049 (closed)