Fix acknowledged functionality in deferrable mode for PubSubPullSensor

[MaksYermak]

In this PR I have refactored the trigger for PubSubPullSensor. It is needed to fix the issue in acknowledged functionality in deferrable mode. We need to acknowledge after the pull, because the pull command generates an acknowledge ID which expires by default after 10 seconds.

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[eladkal]

It is needed to fix the issue in acknowledged functionality in deferrable mode.

If this is a bug fix then we should have a unit test to avoid regression

[MaksYermak]

It is needed to fix the issue in acknowledged functionality in deferrable mode.

If this is a bug fix then we should have a unit test to avoid regression

@eladkal Previously it was a problem in bad trigger design. In that design wasn't included that acknowledge ids exist only 10 seconds after pull. This creates an issue for users when the trigger passes successfully, but on PubSub service in Google Cloud they still have messages. It happens for users, because PubSub's trigger sleep after the pull command and then on the acknowledged stage trigger acknowledges the expired ID. I do not see how we can cover this case by unit test.

[VladaZakharova]

Hi @potiuk @Lee-W @Taragolis @ahidalgob !
Can you please take a look on the changes here? Thanks!