flagging columns null instead of null from table config

[walterddr]

currently enabled null support is a table-level configuration. we need to create column-level null support to indicate whether a column is nullable.

This:

1. enables calcite to generate more efficient plan when column is non-nullable.
2. provide runtime better boxing/unboxing optimization, currently everything need to handle in generic fashion

[walterddr]

CC @Jackie-Jiang @ankitsultana @61yao

[Jackie-Jiang]

We can add a boolean variable _nullable in the FieldSpec to indicate whether we should create NullVector for the column. For backward compatibility, when the table level configuration is enabled, we can set all fields as nullable

[walterddr]

We can add a boolean variable _nullable in the FieldSpec to indicate whether we should create NullVector for the column. For backward compatibility, when the table level configuration is enabled, we can set all fields as nullable

+1 that's a good idea.