add Feature base class to improve support for user-defined features

[alex-hh]

intended as fix for #7220 if this kind of extensibility is something that datasets is willing to support!

class ListOfStrs:
    requires_encoding = True
    def encode_example(self, value):
        if isinstance(value, str):
            return [str]
        else:
            return value
feats = Features(strlist=ListOfStrs())
feats.encode_example({"strlist": "a"})["strlist"] == feats["strlist"].encode_example("a")