Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

Motivation

Python bindings are available since a long time, but only a few operations (operation::on(...) wrappers) were exposed.
There was a great plan to implement feature in python parser that could automatically detect G-API operation (via GAPI_TYPED_KERNEL macro or so) and expose it into python, but this functionality is more about giving an opportunity to user to implement python kernels for already existing in G-API operations.

This PR is going to expose just operation::on wrappers to python in order to give user everything that is available from c++, because now, for developers who don't build from source and change the code only available a small amount of functionality.
Since a lot of developers use opencv from pip let's expose it once and forever.

Great example of G-API usage: https://github.com/xiong-jie-y/g_api_examples
TODO list:

• Expose core
• Expose imgproc
• Expose video
• Expose stereo
• Other stuff (e.g constant initialization for G-Type's & some compiler args)

Problems

Some of imgproc operations are still not exposed because of complexity of wrapping their return types to python.

• GArray<GArray<Point>> findContours(...) - 2 overloads
• std::tuple<GArray<GArray<Point>>,GArray<Vec4i>> findContoursH(...) - 2 overloads
• GOpaque<Vec4f> fitLine2D(...) - 4 overloads
• GOpaque<Vec6f> fitLine3D(...) - 4 overloads
• GMatP NV12toRGBp(...)
• GMatP NV12toBGRp(...)
• GMatP resizeP(...)

Issue: #22736