Verifies that timeout field and fire_at options are valid.
Checks that the field a timeout reads — its field, or its fire_at when it
declares one — references an existing attribute or calculation on the
resource. A fire_at field is read from the trigger's where clause exactly
as a field is, so it is checked exactly as one.
Runs after all transformers have added attributes and calculations to the resource.
Skips field existence validation for the default :state_entered_at since it is always present.