Custom batch settings on a producer

[anguillanneuf]

Would the library support setting my own batch settings on a producer? Right now a default is passed in.

java-pubsublite-kafka/src/main/java/com/google/cloud/pubsublite/kafka/ProducerSettings.java

Lines 57 to 78 in 14a7055

	public Producer<byte[], byte[]> instantiate() throws ApiException {
	PartitionCountWatchingPublisherSettings publisherSettings =
	PartitionCountWatchingPublisherSettings.newBuilder()
	.setTopic(topicPath())
	.setAdminClient(newAdminClient())
	.setPublisherFactory(
	partition -> {
	try {
	return SinglePartitionPublisherBuilder.newBuilder()
	.setServiceClient(
	PublisherServiceClient.create(
	ServiceClients.addDefaultSettings(
	topicPath().location().extractRegion(),
	ServiceClients.addDefaultMetadata(
	PubsubContext.of(FRAMEWORK),
	RoutingMetadata.of(topicPath(), partition),
	PublisherServiceSettings.newBuilder()))))
	.setTopic(topicPath())
	.setPartition(partition)
	.setBatchingSettings(DEFAULT_BATCHING_SETTINGS)
	.build();
	} catch (Throwable t) {