Go SDK should use the combine_globally urn for global combines.

Reported on [https://stackoverflow.com/questions/66446338/issue-with-combine-function-in-apache-beam-go-sdk/66486052#66486052](https://stackoverflow.com/questions/66446338/issue-with-combine-function-in-apache-beam-go-sdk/66486052#66486052)

The root is that the Go SDK doesn't use the "beam:transform:combine_globally:v1" URN, and always uses "beam:transform:combine_per_key:v1" even for global combines, with a AddFixedKey DoFn.

URN in the proto: [https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/beam_runner_api.proto#L347](https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/beam_runner_api.proto#L347) 

Go SDK only having combine_per_key [https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L42](https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L42)

We currently "detect" combines via a CombinePerKey scope [https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/graph/edge.go#L434](https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/graph/edge.go#L434) 
added at beam.TryCombinePerKey 
[https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/combine.go#L58](https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/combine.go#L58)

We convert combines into the CombinePayload here
[https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L253](https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L253)
called above here: 
[https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L241](https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/core/runtime/graphx/translate.go#L241) 

We probably want to just add a graph.CombineGlobal op ( vs the existing combine node), or modify the "CombinePerKey" scope hack to have a CombineCombineGlobal variant, or somehting that is cleaner than currently exists.

We'd also want to make sure the optimization takes place properly, which should be simple enough to detect timing wise at least once, if not as a regular benchmark.

Imported from Jira [BEAM-11928](https://issues.apache.org/jira/browse/BEAM-11928). Original Jira may contain additional context.
Reported by: lostluck.