videoio: add routines to query information about backends API

into cv::videoio_registry namespace

modules/python/src2/cv2.cpp

@@ -1563,8 +1563,6 @@ PyObject* pyopencv_from(const Moments& m)
                          "nu30", m.nu30, "nu21", m.nu21, "nu12", m.nu12, "nu03", m.nu03);
 }
 
-#include "pyopencv_custom_headers.h"
-
 static int OnError(int status, const char *func_name, const char *err_msg, const char *file_name, int line, void *userdata)
 {
     PyGILState_STATE gstate;
@@ -1802,6 +1800,7 @@ static int convert_to_char(PyObject *o, char *dst, const char *name = "no_name")
 #  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
 #endif
 
+#include "pyopencv_custom_headers.h"
 #include "pyopencv_generated_types.h"
 #include "pyopencv_generated_funcs.h"