WIP: [BEAM-5798] Added "withTopicFn()" to set sink topics dynamically

[aromanenko-dev]

Added withTopicFn() to set sink topics dynamically based upon contents of the data.
It's not supported when withEOS() is using since we can't obtain exact number of all partitions in this case.

@rangadi please, take a look

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

[aromanenko-dev]

Retest this please

[lukecwik]

Just waiting on tests to pass, and will merge on green.

[lukecwik]

R: @lukecwik

[rangadi]

I don't think we want to merge this. @aromanenko-dev, could you confirm?

On the mailing list, we agreed that it is better to support writing ProducerRecords, which get all this flexibility and more.

[lukecwik]

Marked with request changes based upon thread feedback.

[lukecwik]

@rangadi Thanks for pointing that out.

[aromanenko-dev]

@lukecwik Yes, we had a discussion about better solution with @rangadi on user mailing list (see: https://lists.apache.org/thread.html/a89d1d32ecdb50c42271e805cc01a651ee3623b4df97c39baf4f2053@%3Cuser.beam.apache.org%3E), so, I think I'll create another PR for this Jira.
Sorry for inconvenience with that.

[lukecwik]

Closed PR as per authors comment.