[Feature Request]: Customize MAX_BATCH_SIZE in DoFn.process_batch

### What would you like to happen?

I'm using a tool in my beam template that benefits largely from batching. In general, the more the better. I noticed that my `DoFn` subclass's `process_batch` is only ever getting 4096 items. This is apparently [hard coded](https://github.com/apache/beam/blob/eed1713b5b8dc8e83ffb1fa43a4d0564083a23ce/sdks/python/apache_beam/runners/worker/operations.py#L262) and can't be changed.

Is there something special about 4096? If not, how difficult would it be to allow users to specify ideal batch sizes?

### Issue Priority

Priority: 3

### Issue Component

Component: sdk-py-core