[pandas 2.0] Removing type_ and length_ members from Array base and pushing them into sub-classes and adding a signature for obtaining covariant return type for DataType

[joshuastorck]

• Removing the DataType as a member of the Array class and adding a new virtual to Array named type_reference that makes it possible for Array sub-classes to have a covariant return type for the DataType
• Removing the length_ member from Array since is doesn't make sense for sub-classes such as CategoryArray, since it can obtain its length from its codes array. This changes the length function to a virtual.
• Changed the signature of Array::type to return a shared_ptr to a const DataType instead of a non-const DataType
• Add shared_ptr members to Array sub-classes that are the expected concrete types: CategoryType, NumericType, etc.

[wesm]

Rebased this and made some minor tweaks. Will merge once the build runs