Support - Batching when producing

As described here:

- https://pulsar.apache.org/docs/en/concepts-messaging/#batching
- https://pulsar.apache.org/docs/en/develop-binary-protocol/#batch-messages
- https://pulsar.apache.org/docs/en/client-libraries-java/#configuring-producers

No file, test, or entry point is named. Start by reading the three linked Apache Pulsar batching and producer documents, then locate the relevant producing path in the client. Done means implementing the requested batching support with behavior consistent with those references and validating it with appropriate tests.

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