Add new index to alert management alerts

What does this MR do and why?

• Adds a new index to alert_management_alerts on project_id, domain, and status to fix the issue raised in Alert Management - getAlertsCount graphQL query... (#500173 - closed) • Max Woolf • 17.7
• Seems that this only affects projects with many alerts, but it makes it unusable for those customers and there's minimal cost to this additional index.

Query plans

Old plan

 HashAggregate  (cost=66466.59..66466.62 rows=3 width=10) (actual time=22729.710..22729.713 rows=3 loops=1)
   Group Key: alert_management_alerts.status
   Buffers: shared hit=5127 read=41960 dirtied=905
   WAL: records=1001 fpi=905 bytes=6238861
   I/O Timings: read=22415.945 write=0.000
   ->  Index Scan using index_alert_management_alerts_on_project_id_and_iid on public.alert_management_alerts  (cost=0.56..66237.76 rows=45765 width=2) (actual time=5.834..22684.086 rows=46736 loops=1)
         Index Cond: (alert_management_alerts.project_id = XXX)
         Filter: (alert_management_alerts.domain = 0)
         Rows Removed by Filter: 0
         Buffers: shared hit=5127 read=41960 dirtied=905
         WAL: records=1001 fpi=905 bytes=6238861
         I/O Timings: read=22415.945 write=0.000

New plan (now index-only scan)

 Aggregate  (cost=0.44..1825.78 rows=3 width=10) (actual time=0.315..12.651 rows=3 loops=1)
   Group Key: alert_management_alerts.status
   Buffers: shared hit=1748 read=43
   I/O Timings: read=0.389 write=0.000
   ->  Index Only Scan using idx_alerts_project_domain_status on public.alert_management_alerts  (cost=0.44..1596.74 rows=45803 width=2) (actual time=0.133..9.681 rows=46736 loops=1)
         Index Cond: ((alert_management_alerts.project_id = XXX) AND (alert_management_alerts.domain = 0))
         Heap Fetches: 2108
         Buffers: shared hit=1748 read=43
         I/O Timings: read=0.389 write=0.000
Settings: seq_page_cost = '4', work_mem = '100MB', effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5'

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Before	After

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Related to #500173 (closed)