API: Add extension type registry to API

[TomAugspurger]

In pandas.api.extensions probably.

[TomAugspurger]

Actually, I prefer something like

def register(cls):
    """Class decorator to register an ExtensionType with pandas.

    This enables operations like ``.astype(name)`` for the name
    of the ExtensionDtype.

    Examples
    --------
    >>> from pandas.api.extensions import register, ExtensionDtype
    >>> @register
    ... class MyExtensionDtype(ExtensionDtype):
    ...     pass
    """
    registry.register(cls)
    return cls

That gives us a bit more freedom in changing how registry is implemented.