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

從 issue 中的 ctypes CFUNCTYPE callback 進入點和 C variadic callback 範例開始。釐清 variadic 引數所支援的表示方式和行為，然後決定需要哪些測試來證明定義和呼叫這類 callback 能夠正常運作。