[BEAM-10289] Adding required transform ID to Go channel split.

[youngoli]

Not much to elaborate on. This TransformID is technically required by the proto but wasn't used until now, but I ran into some problems when using it with the FnAPI runner, so I'm adding it.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang		---		---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

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[youngoli]

R: @lostluck

[lostluck]

Thanks!