Add common.compat alias for BaseEventTrigger

[amoghrajesh]

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Each provider repeats a variant of:

if AIRFLOW_V_3_0_PLUS:
    from airflow.triggers.base import BaseEventTrigger, TriggerEvent
else:
    from airflow.triggers.base import (  # type: ignore[assignment]
        BaseTrigger as BaseEventTrigger,
        TriggerEvent,
    )

Several of these also drag TriggerEvent into the version branch even though it exists in both 2.x and 3.x, and pull in an AIRFLOW_V_3_0_PLUS import that isn't used elsewhere.

I am proposing that we add a airflow.providers.common.compat.triggers, which resolves BaseEventTrigger through the existing create_module_getattr rename machinery and falls back to BaseTrigger on Airflow 2.x. Call sites collapse to a single import.

The iceberg trigger is migrated as the first consumer: #71387

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