Alert Management - Ingest severity by Alerts endpoint

What does this MR do?

This ensures that we save the severity of alerts properly.

Severity is an expected param as of the Alert Management Epic.

If the user doesn't provide the severity then we use the default, which is critical.

I also refactored some specs to ensure we are covering all the fields which are user settable via the endpoint.

Note: Changelog not needed as this is a backstage change on a feature not yet enabled.

Conformity

• [-] Changelog entry since behind feature flag
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Closes #217048 (closed)