Support configuring the BatcherBuilder through a producer property

**Is your enhancement request related to a problem? Please describe.**
Consumers support `KEY_SHARED` subscriptions. For this to work with producer batching, the producer needs to use a `KEY_BASED` `BatcherBuilder`. The `subscriptionType` of the consumer is currently configurable through properties. The `BatcherBuilder` used by the producer however, is only configurable through code. It would be nice if this could also be done by property.

**Describe the solution you'd like**
Have a configuration property (e.g. `batcherBuilderType`) which supports values `DEFAULT` and `KEY_BASED` for configuring the `BatcherBuilder` used by the producer.

**Describe alternatives you've considered**
None

**Additional context**
Third party products that use pulsar (e.g. debezium) expose the pulsar config through properties. Only being able to configure this in-code is inconvenient in such cases.

No files or tests are named. Start by tracing how producer properties and the producer's BatcherBuilder are currently configured, alongside the existing consumer subscriptionType property handling. Done means a producer property accepts DEFAULT and KEY_BASED and selects the corresponding BatcherBuilder, with coverage for both values.

Written by the indexing model from the issue text.