[Filebeat] Update Google Pub/Sub Input to wait for publisher ACK

[andrewkroh]

The Google Pub/Sub input needs to be updated to wait for the ACK from the internal libbeat publisher before ACK'ing the pub/sub message.

The current implementation ACKs the message after the message has been accepted by the internal queue, but not when the internal publisher notifies the publisher client's ACKed callback.

[andrewkroh]

As an example the S3 input is ACK'ing its message after the callback from the libbeat publisher:

beats/x-pack/filebeat/input/s3/input.go

Lines 141 to 147 in 248db1c

	ACKEvents: func(privates []interface{}) {
	for _, private := range privates {
	if s3Context, ok := private.(*s3Context); ok {
	s3Context.done()
	}
	}
	},

[andrewkroh]

@adriansr Can you please remove any beta labels from code/docs for the input type after you address this issue.