[protocol] Add a ColumnSchema abstraction? #253

class ColumnSchema(TypedDict):
 # the underlying physical representation
 dtype: DType
 # if the column is categorical, describes how to interpret the contents
 categorical_encoding: Optional[CategoricalDescription]
 # if the column supports null values, describes how they are represented
 null_encoding: Optional[Tuple[ColumnNullType, Any]]
 # arbitrary metadata attached to the column, possibly empty
 metadata: Dict[str, Any]
class Column(ABC):
 ...
 @property
 @abstractmethod
 def schema(self) -> ColumnSchema: ...