ENH: Add infrastructure to simplify unicode/StringDType promoters

[ngoldbaum]

Originally from a comment by @seberg: #27636 (review)

The one story that I could potentially see is to make something like an abstract (doesn't need to be an actual thing could just be a tuple as in isinstance, but conceptually):

class StringOrUnicodeDType:

so that we could just add one promoter, but the promoter would be need to programmatically decide whether the result should be unicode or string. (Similar to the "this dtype must occur", style of logic. But these ufuncs that include ints show that just a list of dtypes is not super helpful a such)