[BEAM-10139] Add Python cross-language wrapper for Spanner Java Write

[pjotrekk]

R: @chamikaramj

I'm not sure how should I register the mutation - I suppose that the user will have to register 3 named tuples: keyset, mutation and row, but I'll investigate it further. I'm open to suggestions in this matter.

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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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