Add support for defining variadic callbacks via ctypes

Feature or enhancement

Proposal:

When defining a callback with ctypes (CFUNCTYPE) it would be helpful to be able to replicate variadic functionality
for instance, take the following c code

void FormatPrintf(datap* format, char* fmt, ...) {

    va_list args;
    int length = 0;

    va_start(args, fmt);
    length = vsnprintf(NULL, 0, fmt, args);
    va_end(args);
    if (format->length + length > format->size) {
        return;
    }

    va_start(args, fmt);
    (void)vsnprintf(format->buffer, length, fmt, args);
    va_end(args);
    return;
}

Making something compatible with ctypes

@CFUNCTYPE(None, ctypes.POINTER(datap), ctypes.c_char_p, ctypes.c_variadic)
def FormatPrintf(format, fmt, variadic)
    args = variadic.va_list
    ...

Maybe even could be handled as a pyobject list of ctypes objects as the variadic on the python side. Not sure what all the options are explicitly.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Comece pelo ponto de entrada do callback ctypes CFUNCTYPE e pelo exemplo de callback variádico em C no issue. Esclareça a representação e o comportamento compatíveis com argumentos variádicos e, em seguida, determine quais testes são necessários para mostrar que definir e invocar um callback desse tipo funciona.