Move PrometheusAlert from EE to CE

Summary

Contributes to #42640 (closed)

Move PrometheusAlert 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

PrometheusAlert class, factory, and associations: project, environment, prometheus_metric

Optional: Intended side effects

Optional: Missing test coverage