[BEAM-6979] Add support for double well known coder to Java and add tests for Java & Python

[lukecwik]

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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.

[lukecwik]

R: @robertwb
CC: @boyuanzz@laschmidt

[robertwb]

Looks good. Thanks!