[Feat][Format] Support nullable key-value in property metadata

[acezen]

Is your feature request related to a problem? Please describe.
Some graph database(like HugeGraph) that they have a metadata of property that show the property can have null value or not. GraphAr should support the metadata.

Describe the solution you'd like
Add a key call nullable to Property struct and provide related API.
like is_primary

Describe alternatives you've considered
no alternatives

Additional context
related discussion #343

[Thespica]

Should the new field be alternative when parsing?

If it is required, we have to change all data, and loss compatibility backwards.

[acezen]

Should the new field be alternative when parsing?

If it is required, we have to change all data, and loss compatibility backwards.

yes, I think it can be optional in info yaml that if the property don't contain nullable key, it's false by default.

[acezen]

hi, @Thespica , since the C++ library has support the feature, can you help to support the nullable key-value for GraphAr-Spark library too.

[SemyonSinchenko]

@acezen May we wait with it a little? Cause I'm making a huge refactoring of graphar-spark (#320, #361) and merging will be pain

[acezen]

@acezen May we wait with it a little? Cause I'm making a huge refactoring of graphar-spark (#320, #361) and merging will be pain

yes, this issue can wait until the refactor merged

[acezen]

closed as the issue has been resolved.