Rename get_dtype_of_same_kind to get_dtype_of_same_variant and move to converters.py

[mroeschke]

Description

As we're working toward enabling native support for pandas extension types, #21229, we'll need to make heavy use of get_dtype_of_same_kind to return an extension type if the input has an extension type.

Similar to #21281, I think it's good opportunity to move this function to it's own dtype/converters.py file, rename it to get_dtype_of_same_variant, and finish out it's implementation by using functools.singledispatch to define all the conversions between dtype variants.

Ops-Bot-Merge-Barrier: true

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

I think the PR over-zealously assumes that an API will return the same variant as itself. But for example a pd.StringDtype may return a int64 and not an pd.Int64Dtype, so I may need to rethink this