[BP-2.0][FLINK-37301][state/forst] Fix wrong flag returned by ForStStateBackend#supportsSavepointFormat

[Zakelly]

What is the purpose of the change

The ForStStateBackend did not override the supportsSavepointFormat, which means it gives the wrong flag. We should fix this.

Brief change log

• Return true in supportsSavepointFormat when it is NATIVE format

Verifying this change

This change is a trivial rework, a simple test is added.

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: no

Documentation

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

[flinkbot]

CI report:

• edf3608 Azure: SUCCESS

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