Support callable fix_values in _ValidHDU.req_cards

[tbabej]

This is a follow up for #5935.

The API of the _ValidHDU.req_cards method requires a fix_value, which is then, however, encapsulated inside a function (see code snipped below). In some cases (i.e. #5935) a dynamic fix_value is useful, but to do that one has to precompute it using a function call, only to have the result encapsulated inside another function.

From astropy/io/fits/hdu/base.py:1155

    if fixable:
        def fix(self=self, keyword=keyword, val=fix_value):
            self._header[keyword] = fix_value

    errs.append(self.run_option(option, err_text=err_text,
                fix_text=fix_text, fix=fix, fixable=fixable))

Instead of performing this convoluted function wrapping-unwrapping the req_cards could check if fix_value is a callable, and if it is, it would be interpreted as the method performing the fix itself.