Expose Deep sleep wakeup cause

Esp32 can tell what was the cause of deep sleep wakeup. This is being used already internally to figure out different timeouts based on the cause, but it would be nice to expose this to an automation in on_boot.
Example use case: battery powered rain gauge, when it is woken up by GPIO because the bucket was tripping, capture rainfall data, when it is waken up by timeout, just measure temperature, wind speed, direction etc.