This PR implements Google Managed Kafka (GMK) backend support for the Python Pub/Sub Lite client.

It matches the Go implementation by adding:

1. Gating to PublisherClient and SubscriberClient to detect MessagingBackend.MANAGED_KAFKA and route to Kafka-based implementations.
2. KafkaPublisherClient (sync & async) using confluent_kafka.Producer.
3. KafkaAsyncSingleSubscriber (used in multiplexed subscriber clients) using confluent_kafka.Consumer with thread-safe offset commits.
4. Automatic consumer group ID derivation from Pub/Sub Lite subscription path.
5. Header translation including event time parsing and multi-valued header flattening.
6. OAuth callback support using Google credentials.

Tested thoroughly with unit tests mocking confluent_kafka.