[FLINK-14794] Fixing KeyedStream#transform to properly relay type information

[AHeise]

What is the purpose of the change

Fixing KeyedStream#transform to properly relay type information if invoked with factories.

Brief change log

• Overloading the correct function of DataStream in KeyedStream, such that the type information is passed to both operator and factory based transformation methods.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

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Last check on commit d76f653 (Fri Feb 28 21:47:00 UTC 2020)

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[igalshilman]

Hi @AHeise, I've built your PR locally and validated that indeed I can use an *StreamOperatorFactory on a KeyedStream and it works as expected.