Add Alerts Service to Projects - backend

What does this MR do?

This MRs adds "Alerts Service" to Projects supporting the frontend https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15733.

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/13203

Feature flag generic_alert_endpoint + GitLab Ultimate

Enable feature flag via

echo 'Feature.enable(:generic_alert_endpoint)' | bundle exec rails c

Screenshots

Service list	Service view	Reset key

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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