Minimal GraphQL for Alert Management Alerts

What does this MR do?

This adds a GraphQL typer for accessing AlertManagement Alerts.

The resolver expects either no params, in which case all the alerts for the project will be returned. If an iid param is given then a single Alert is returned (single item in the array).

Sorting, Filtering, Pagination will come in subsequent MRs.

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 #213880 (closed)