GroupIntoBatches transform for Go SDK

[damccorm]

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

 

And Python counterpart:

beam/sdks/python/apache_beam/transforms/util.py

Line 684 in c445fdf

class GroupIntoBatches(PTransform):

Imported from Jira BEAM-8272. Original Jira may contain additional context.
Reported by: johnpatoch69.