[FLINK-36072][runtime] Sort-merge shuffle supports accumulating small buffers to reduce network overhead.

[JunRuiLee]

What is the purpose of the change

https://issues.apache.org/jira/browse/FLINK-36072
Optimizing the Overhead of the Network Layer in Adaptive Execution Scenarios

Brief change log

• On the downstream side, enable a single input gate to support a single input channel consuming multiple subpartitions.
• On the upstream side, enable sort-merge shuffle could use single partition reader to read multiple subpartitions
• On the upstream side, enable sort-merge shuffle supports accumulating small buffers to reduce network overhead.

Verifying this change

This change added tests and can be verified such as CreditBasedPartitionRequestClientHandlerTest.

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

[flinkbot]

CI report:

• 8fdcf26 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[davidradl]

Reviewed by Chi on 05/12/24 in hand

[JunRuiLee]

Thanks @reswqa for reviewing this PR. I have updated it accordingly and rebased it onto the master. PTAL.

[JunRuiLee]

@flinkbot run azure

[reswqa]

Thanks for the update, LGTM.