Add search to Alert Management Alerts GraphQL query

What does this MR do?

This adds the ability to Search for Alerts in the Alert Management GraphQL query.

It does this via the AlertsFinder using the search scope on AlertManagement::Alert.

Query plan

Query plan is here https://explain.depesz.com/s/QMAz

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

Part of #213884 (closed)