feat(PubSub): Add update_topic_type sample

[cy-yun]

b/322221088

[snippet-bot]

Here is the summary of changes.

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• pubsub/api/src/update_topic_type.php:27, tag pubsub_update_topic_type

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[bshaffer]

This is a valid way to do it, but I think when updating, the API would normally look a bit more like this:

Suggested change

	$topic = $pubsub->topic($topicName, [
	'ingestionDataSourceSettings' => [
	'aws_kinesis' => [
	'stream_arn' => $streamArn,
	'consumer_arn' => $consumerArn,
	'aws_role_arn' => $awsRoleArn,
	'gcp_service_account' => $gcpServiceAccount
	]
	]
	]);
	$topic->update([], [
	'updateMask' => [ 'ingestionDataSourceSettings' ]
	]);
	$topic = $pubsub->topic($topicName);
	$topic->update([
	'ingestionDataSourceSettings' => [
	'aws_kinesis' => [
	'stream_arn' => $streamArn,
	'consumer_arn' => $consumerArn,
	'aws_role_arn' => $awsRoleArn,
	'gcp_service_account' => $gcpServiceAccount
	]
	]
	], [
	'updateMask' => [
	'ingestionDataSourceSettings'
	]
	]);