[WIP] Flink unified translation

[jto]

This PR provides a new "unified" implementation of a translator for Flink capable of translating all workflows, portable or not, streaming and batch.

It replaced the 4 implementations that currently exist with the following advantages:

• Uses the new DataStream API rather that the deprecated DataSet.
• Uses the new Flink source implementation rather that the deprecated InputFormat and SourceFunction
• Easier to maintain. There's a lot of overlap between the existing implementation and code is largely copy pasted from each other (with subtle accidental variations)

fixes #28617

• Use Unified translator for native streaming pipelines
• Use Unified translator for native batch pipelines when --useDataStreamForBatch is passed
• Use Unified translator for portable streaming pipelines
• Use Unified translator for portable batch pipelines

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[jto]

There's currently an issue in Batch mode specifically with timers because of a race condition in ExecutableStageDoFnOperator.

Because processElement is async and the actual waiting for bundle being processed only happens in finishBundle, the following issue 2 issues will happen:

• Timers need to be forcibly triggered at the end of the bundle in batch mode otherwise they may be lost. This is easily fixed because a similar hack exists for processing time events.

• A DoFn that sets a timer will execute and its corresponding bundle will actually finish, however, while the DoFn processElement is successfully executed the actual registration of the timer is not yet completed (everything is async since the body of the DoFn runs in a different process / thread). The following events will happen:

  1. Bundle finishes,
  2. Timers are set and fired,
  3. next bundle is started.

  Because the registering and firing of timers happens BETWEEN 2 bundles (after finish, before start) those timers firing will just be lost.

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