[BEAM-8354] Add RowJsonSerializer

[TheNeuralBit]

Adds a RowJsonSerializer, and renames RowJsonDeserializerTest to RowJsonTest, as it now includes roundtrip tests from row to json and back.

R: @apilloud

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.

[kennknowles]

This looks good. There's a Java tradition (which we don't follow all the time) of putting all tests for Foo in FooTest. It might make sense to consolidate the deserializer and serializer into an aggregating class just to make this more "normal".

[TheNeuralBit]

I like the idea of combining serializer and seserializer into an outer class. Would there be any concern with changing the API since RowJsonDeserializer has been public for a while? Or is it generally considered ok to change around classes in util?

[kennknowles]

Ah, everything in the util package is not for users. (they may or may not be on board with this design decision)