fix: [PubSub] encode option

[Seb33300]

After we upgraded from v1 to v2, the consume() method is not longer returning the message data decoded.

Before, it was done by setting the encode option here:

google-cloud-php-pubsub/src/PubSubClient.php

Lines 161 to 167 in d95e398

	if ($connectionType === 'grpc') {
	$this->connection = new Grpc($this->configureAuthentication($config));
	$this->encode = false;
	} else {
	$this->connection = new Rest($this->configureAuthentication($config));
	$this->encode = true;
	}

It looks like the encode is never set, and there is no way to set it anymore.

I am not sure if this sis the correct fix for this issue.

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