Support SortValues GroupByKey extension

A port of the Java [SortValues](https://beam.apache.org/documentation/sdks/java-extensions/) extension would be helpful in the Go SDK.

Brief summary of the extension: for a GroupByKey or CoGroupByKey operation, allow the user to specify that the value streams per key should be sorted (either by their byte encoding or by a secondary sort key) upon delivery the user's ParDo.

Imported from Jira [BEAM-4405](https://issues.apache.org/jira/browse/BEAM-4405). Original Jira may contain additional context.
Reported by: schroederc.

Start with the Go SDK's GroupByKey and CoGroupByKey entry points, then compare the Java SortValues extension linked in the issue and consult the imported BEAM-4405 context. Define how byte-encoding and secondary-key ordering should be exposed and delivered to ParDo users; done means the behavior is implemented and covered for both grouping operations.

Written by the indexing model from the issue text.