Please add the Findcontour feature

[Trojan9]

Is the requested feature available in the C++/Java/Python interfaces for OpenCV?
If yes, please describe the function/method & class that contains the feature. If not, please explain why Flutter/Dart requires said feature nonetheless.

Describe the solution you'd like
A clear and concise description of what you want to happen. Provide links to existing documentation, if possible.

Describe alternatives you've considered
Have you tried any alternative solutions? If yes, please provide links to the same. If not, can the same feature be achieved by cascading the existing set of effects/methods available in the latest build?

Additional context
Add any other context or screenshots about the feature request here.

[Trojan9]

https://docs.opencv.org/3.4/df/d0d/tutorial_find_contours.html

That is the link to the feature