[BEAM-8096] Allow runner to configure "subnetwork"

[jackwhelpton]

Enables the subnetwork to be configured for the Dataflow runner when using the Go SDK, by analogy with the same functionality for Java and Python. The flag name matches that for Python:

go run src.go \
  --runner dataflow \
  ...
  --network {network} \
  --subnetwork regions/{region}/subnetworks/{subnetwork}
  ...

---

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---	---

Pre-Commit Tests Status (on master branch)

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

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[jackwhelpton]

R: @DanOliveira

[youngoli]

Looks good to me.

[lostluck]

LGTM