Add generic typing to Arrowbic base extension type

[balancap]

Adding generic typing is improving the general code quality, as well as, making usability of the library better in common IDEs (VS code, ...)

[balancap]

Currently blocked by issue in PyArrow implementation: it does not support generic extension type, i.e.

TItem = TypeVar("TItem")

class BaseExtensionType(pa.ExtensionType, Generic[TItem]):
    pass

will lead a failure when calling pa.ExtensionArray.from_storage factory method.

Implementing the generic feature only for ExtensionArray at the moment, in #18