[Feat][Format][Spark] Add nullable key in meta-data

[Thespica]

Proposed changes

source code was done. Besides, should update test data with is_nullable declared directly.

Checklist

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Further comments

related issue #351

[acezen]

Does the implementation can parse the is_nullable from yaml file? Can you add a test to check that?

[acezen]

hi john，the #369 has been merged, you can rebase the main.

[acezen]

LGTM