Refactor Trigger to be an interface and update Monitor

[qreshi]

Issue #, if available:

Description of changes: For Document-Level Alerting, a new Trigger type will be introduced. To scale for this change, Trigger was refactored to be an interface. The current Trigger is now referred to as TraditionalTrigger.

Monitor now has monitor_types where the current Monitor is referred to as a traditional_monitor and uses TraditionalTriggers. The aggregation/multi-alert Alerting behavior will be available in aggregation_monitors which will use AggregationTriggers. These names are not set in stone, it was mostly to differentiate the implementations.

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[lezzago]

We need to remember to update the schema version once we are merging into main