[BEAM-7023] WithKeys PTransform for Python SDK

[ttanay]

A PTransform that takes a PCollection, and either a constant key or a
callable, and returns a PCollection of (K, V), where each of the values in
the input PCollection has been paired with either the constant key or a key
computed from the value.

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Post-Commit Tests Status (on master branch)

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

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.

[ttanay]

Run Python_PVR_Flink PreCommit

[ttanay]

R: @robinyqiu @aaltay @pabloem

[pabloem]

LGTM : ) thanks Tanay!