GroupIntoBatches transform for Go SDK

Add a PTransform that batches inputs to a desired batch size. Batches will contain only elements of a single key.

It should offer the same API as its Java counterpart:

[https://github.com/apache/beam/blob/11a977b8b26eff2274d706541127c19dc93131a2/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/GroupIntoBatches.java](https://github.com/apache/beam/blob/11a977b8b26eff2274d706541127c19dc93131a2/sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/GroupIntoBatches.java)

And Python counterpart:

https://github.com/apache/beam/blob/c445fdfdfab4a191aa780210564199f2873f85d8/sdks/python/apache_beam/transforms/util.py#L684

Imported from Jira [BEAM-8272](https://issues.apache.org/jira/browse/BEAM-8272). Original Jira may contain additional context.
Reported by: johnpatoch69.