[Example/OpenCV] Add OpenCV custom filter example

[Example/OpenCV] Add OpenCV custom filter example

This patch newly adds the OpenCV custom filter example, which scales an
input tensor of [N][input_h][input_w][M] to an ouput tensor of
[N][output_h][output_w][M]. The shape of an output tensor could set by
custom property as "custom=[new-x]x[new-y]", where new-x and new-y are
unsigned integers.

This example is developed based on
nnstreamer_customfilter_example_scaler_allocator.c file.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>