[FLINK-19434][DataStream API] Add source input chaining to StreamingJobGraphGenerator

[StephanEwen]

Prerequisites

This PR is based on #13512 , on which it relies as a testing util.

What is the purpose of the change

This adds the StreamingJobGraphGenerator logic for chaining sources to N-Ary-Input Operators.
Joint work with @tsreaper.

Verifying this change

This change adds the SourceChainingITCase to verify correctness.
The fact that the changes do not interfere with other stream graph setup logic is covered by the existing StreamingJobGraphGeneratorTest and the sum of all end-to-end integration tests that execute DataStream and Table programs.

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): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/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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Automated Checks

Last check on commit 73ff86c (Tue Sep 29 21:41:53 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• 7e1dda8 Azure: FAILURE

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[StephanEwen]

The discussion / review for this PR happened on feature branches between the developers.
We worked directly on a feature, because the branch included some preliminary code to enable this feature for the Blink query engine and use the TPC_DS workload for testing.