Add TO_STRING capability to Go SDK

Go SDK doesn't yet implement TO_STRING, which is necessary for runner hotkey debugging among other things.

URN description:
https://github.com/apache/beam/blob/master/model/pipeline/src/main/proto/org/apache/beam/model/pipeline/v1/beam_runner_api.proto#L329

Involves adding the capability URN "beam:transform:to_string:v1" to the goCapabilities list in graphx/translate.go and the paired handling in exec/translate.go along with the appropriate node to convert elements to strings using fmt.

Using fmt allows user element types to have String() implemented, which will then appear as normal. However, we should also print out the %T for the type, to make it unambiguous for users to know the type of the hot key, which will reduce debugging turn around.

AFAIK only used with Dataflow when "Hot Key Logging" https://cloud.devsite.corp.google.com/dataflow/docs/reference/pipeline-options#debugging is enabled, and there's no closed form runner test for this at present.