[BEAM-7260] UTF8 coder is breaking dataflow tests

[ihji]

StringUtf8Coder is now ModelCoder in JavaSDK. We need to add it to
WELL_KNOWN_CODER_TYPES for dataflow worker harness as well.

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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	---		---	---

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[ihji]

R: @robertwb

[angoenka]

I am optimistically going to merge this change unbroken master.
@robertwb Please review it and if needed we can revert it this PR.

[robertwb]

Thanks. LGTM.