Send email notifications for Generic Alerts

What does this MR do?

This adds the functionality of sending an alert email when a generic (non-prometheus) alert has been received to the alert endpoint.

This brings us to feature parity with Prometheus alerts as far as emails go.

Screenshots

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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

Closes #199400 (closed)