[Feature Request]: Implement BatchElements in the beam-java-sdk

### What would you like to happen?

Beam's python SDK supports [BatchElements](https://beam.apache.org/releases/pydoc/current/apache_beam.transforms.util.html?highlight=batchelements#apache_beam.transforms.util.BatchElements) that batches elements without having to be keyed like in [GroupIntoBatches](https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/GroupIntoBatches.html)

This API is not available in the Java SDK but may be helpful, eg. to issue batch requests to external services over the network when there are no limitations on the maximum number of connections

### Issue Priority

Priority: 3

### Issue Component

Component: sdk-java-core