Allow generic endpoint to receive alerts from external Prometheus

What does this MR do?

This MR enables the generic endpoint to handle Prometheus specific payload so users are only need to enable Settings > Integration > Alerts endpoint and use its token in their Prometheus Alertmanager. No need to enable Settings > Integrations > Prometheus.

Please see #218560 (closed) for more details.

Screenshots

Alerts endpoint enabled
Prometheus Alertmanager config
Prometheus integration disabled
Alerts list
Alerts details

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