feat: add pubsublite-kafka-auth module

[dpcollins-google]

This allows for auth through kafka params.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• samples/snapshot/pom.xml
• samples/snippets/pom.xml

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/pubsublite/KafkaProducerExample.java:19, tag pubsublite_kafka_native_producer

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[anguillanneuf]

I did a pass. Other than the inline comments, could you add a short description for the Python script either in the README or in the file itself?

You probably need to break out the samples into a different PR after releasing the library with the change.