also resolves #18773

Intro

This PR brings support of generic in-graph metadata in the G-API.
The metadata comes into graph with its input arguments and propagates through the graph then.

The main idea behind this metadata today is to tag every object with extra streaming-related info (e.g., a frame number and a timestamp) to allow app-side data synchronization when streaming::desync() feature is used (see tests).

Overview of changes

• cv::GRunArg is not an alias over variant<> anymore, now it is a distinct type;
• cv::GRunArg is extended with a metadata dictionary;
• cv::GCaptureSource is extended to put the meta information to the captured frames;
• Introduced a generic streaming::meta<T>(obj, str) operation which accesses a metadata field str of object obj and of the given type T; its return value is GOpaque<T> which can be later used in the graph;
• Introduced an internal backend (in backends/common) which implements this generic meta<>() operation;
• Implemented a simple metadata propagation model at the GExecutor/GStreamingExecutor level. This is the most debatable part of the patch -- now asynchronous islands have to manage and post this metadata information for outputs on their own. However, there is a default implementation for the synchronous islands (which covers 100% of our public backends).

Sporadic failures in desync* tests

There was an issue with some tests on desync failing in different configurations. The failing check compared number of frames received through the "main" path against the number of frames received through a "desynchronized" path, what in theory shouldn't happen.

The issue occurred due to the main path actually running slower than a desync path on a particular machine or in a particular build configuration. Because of that, a desync path has got its "Stop" message earlier and broadcasted it down to the output queue while the "main" path was still busy. It led to terminating the streaming pipeline earlier that expected and that "counter mismatch" problem occurred because of that.

• 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 Mac
build_gapi_standalone:Linux x64=ade-0.1.1f
build_gapi_standalone:Win64=ade-0.1.1f
build_gapi_standalone:Mac=ade-0.1.1f
build_gapi_standalone:Linux x64 Debug=ade-0.1.1f
Xbuild_image:Custom=centos:7
Xbuildworker:Custom=linux-1
build_gapi_standalone:Custom=ade-0.1.1f
build_image:Custom=ubuntu-openvino-2021.1.0:18.04
build_image:Custom Win=openvino-2021.1.0
build_image:Custom Mac=openvino-2021.1.0
test_modules:Custom=gapi
test_modules:Custom Win=gapi
test_modules:Custom Mac=gapi
buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*