This patch is mainly upstreaming of a long-live internal branch.
The functionality has been tested already in the "real world" scenarios.

• desync() is a new (and for now, the only one) intrinsic
  which splits the graph execution into asynchronous parts
  when running in Streaming mode;

• desync() makes no effect when compiling in Traditional mode;

• Added tests on desync() working in various scenarios;

• Extended GStreamingExecutor to support desync(); also extended
  GStreamingCompiled() with a new version of pull() returning a
  vector of optional values;

• Fixed various issues with storing the type information & proper
  construction callbacks for GArray<> and GOpaque;

• Introduced a new infer(Roi,GMat) overload with a sample;

• Introduced an internal API for Islands to control fusion
  procedure (to fuse or not to fuse);

• Introduced handleStopStream() callback for island executables;

• Added GCompileArgs to metadata of the graph (required for other
  features).

Pull Request Readiness Checklist

See details athttps://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 other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to 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

force_builders=Custom,Custom Win,Custom Macbuild_gapi_standalone:Linux x64=ade-0.1.1fbuild_gapi_standalone:Win64=ade-0.1.1fbuild_gapi_standalone:Mac=ade-0.1.1fbuild_gapi_standalone:Linux x64 Debug=ade-0.1.1fXbuild_image:Custom=centos:7Xbuildworker:Custom=linux-1build_gapi_standalone:Custom=ade-0.1.1fbuild_image:Custom=ubuntu-openvino-2020.3.0:16.04build_image:Custom Win=openvino-2020.3.0build_image:Custom Mac=openvino-2020.3.0test_modules:Custom=gapitest_modules:Custom Win=gapitest_modules:Custom Mac=gapibuildworker:Custom=linux-1# disabled due high memory usage: test_opencl:Custom=ONtest_opencl:Custom=OFFtest_bigdata:Custom=1test_filter:Custom=*