[decoder] Add "tracking" feature for bounding boxes

[decoder] Add "tracking" feature for bounding boxes

- If the property is given as  `... option6=1 ...`, result bounding boxes will be marked with unique IDs.
- Centroid based track algorithm is used. It calculate every euclidean centroid distance among previous boxes and incoming boxes. And match from the least distance pair.

NOTE that this is too naive and trivial to be applied to production level application. Use it for test purpose.

If you need more information about object tracking task, refer https://docs.nvidia.com/metropolis/deepstream/dev-guide/text/DS_plugin_gst-nvtracker.html

Signed-off-by: Yongjoo Ahn <yongjoo1.ahn@samsung.com>