Add alert counts by status to graphql

What does this MR do?

Adds alert counts by status (or category of statuses).

Related issue: #217670 (closed)

Explain output for new finder method:

GroupAggregate  (cost=4.04..4.06 rows=1 width=10) (actual time=0.132..0.133 rows=1 loops=1)
  Group Key: status
  Buffers: shared hit=3 read=1
  I/O Timings: read=0.017
  ->  Sort  (cost=4.04..4.04 rows=2 width=2) (actual time=0.128..0.129 rows=2 loops=1)
        Sort Key: status
        Sort Method: quicksort  Memory: 25kB
        Buffers: shared hit=3 read=1
        I/O Timings: read=0.017
        ->  Seq Scan on alert_management_alerts  (cost=0.00..4.03 rows=2 width=2) (actual time=0.102..0.103 rows=2 loops=1)
              Filter: (project_id = 14986497)
              Buffers: shared read=1
              I/O Timings: read=0.017
Planning Time: 4.155 ms
Execution Time: 0.350 ms

^ Ran against the monitor Tanuki Inc demo project, as it's the only one this feature is currently enabled for.

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team