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

从 issue 中的 ctypes CFUNCTYPE callback 入口点和 C variadic callback 示例开始。明确 variadic 参数所支持的表示形式和行为，然后确定需要哪些测试来证明定义和调用此类 callback 能够正常工作。