[FLINK-22133][core] Add checkpointID to 'SplitEnumerator.snapshotState()'

[StephanEwen]

What is the purpose of the change

This adds a 'checkpointID' to the method SplitEnumerator.snapshotState(), so that connectors against checkpoint-aware systems get the information what checkpoint the snapshot is for.

This is an API breaking change in the SplitEnumerator class

The class is @PublicEvolving, introduced in 1.12 - I opted for doing this change for 1.13 because this was the first released version (labaled BETA in the feature radar) and we avoid instant tech debt this way.

Starting from 1.13 or 1.14, we should mark this API at @Public and thus stable.

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, 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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Last check on commit 935b887 (Sat Aug 28 11:16:05 UTC 2021)

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[dawidwys]

Change itself looks good. +1 assuming all interested parties are fine with breaking the API (I am ;) )

[dawidwys]

Suggested change

	* in the snapshot any more.
	* in the snapshot anymore.

[dawidwys]

Thanks for extending the javadoc! That is really helpful!

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CI report:

• 935b887 Azure: FAILURE

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