[FLINK-39900] [forst] Add configuration option for setting the number of ForSt transfer threads

[francis-a]

What is the purpose of the change

Currently the number of threads used by the ForStStateDataTransfer class is hardcoded to 4. This PR proposes a new state.backend.forst.checkpoint.transfer.thread.num configuration option that allows for providing a custom thread number value.

Brief change log

• Added a new state.backend.forst.checkpoint.transfer.thread.num config option
• Updated the ForStResourceContainer with a getter to return this option value
• Updated ForStStateDataTransfer to provide the new config option value instead of the hardcoded default of 4
• Removed the hardcoded default

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): / 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, ZooKeeper: no
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? Documentation has been added using the existing ConfigOption class

[flinkbot]

CI report:

• e1caf68 Azure: SUCCESS

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