Move Alert class from EE to CE

Summary

Contributes to #42640 (closed)

Move Alert class with its dependencies to CE

Improvements

Moving the generic alerts endpoint feature from EE to CE requires a lot of code changes. All the required changes can be split and moved in several steps (as described here).

Risks

Involved components

• Alert class
• Alert factory
• Alerting::AlertAnnotation class
• Prometheus::AlertFinder class
• Prometheus::AlertPresenter class

Optional: Intended side effects

Optional: Missing test coverage