[BEAM-8139] Implement Spark portable job jars.

[ibzib]

This is a pretty straightforward copy/paste/find/replace of the Flink implementation.

For now, jars must be created by Java. The next step would be to create an equivalent for FlinkUberJarJobServer, though we probably want to fix that first before porting it to Spark.

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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.

[ibzib]

Test failures (:runners:direct-java:needsRunnerTests and :runners:samza:test) look unrelated.

[mxm]

LGTM Perhaps let @iemejia also have a look.