Draft: Add alert events tables + writer + query API

• Adds alert_events table to store raw alert events.
• Adds alerts table to store an alert which is built by aggregating over events having same key of (TenantId, ProjectId, AggTimestamp, AlertType).
• Adds materialized view alerts_mv which pulls events from alert_events and writes into alerts after aggregating over (TenantId, ProjectId, AggTimestamp, AlertType).
• Aggregation window is minutely - as rate limiting windows are also per minute.
• Adds AlertWriter for writing alert events. It uses async insertion for simplicity instead of keeping an internal batch of events to flush.
• Adds AlertsQuerier to allow querying alerts. It takes TenantId and list of ProjectId as parameters.
• Adds /v3/query/alerts path on the query API that hooks on AlertsQuerier.
• Adds alertWriter on rate limiting handler to create and write alert events. (some pieces are WIP)

Related to #2813.