Merge pull request #24178 from dmatveev:dm/streaming_queue

G-API: Introduce a Queue Source #24178

- Added a new IStreamSource class: in fact, a wrapper over a concurrent queue;
- Added minimal example on how it can be used;
- Extended IStreamSource with optional "halt" interface to break the blocking calls in the emitter threads when required to stop.
- Introduced a QueueInput class which allows to pass the whole graph's input vector at once. In fact it is a thin wrapper atop of individual Queue Sources.

There is a hidden trap found with our type system as described in https://github.com/orgs/g-api-org/discussions/2

While it works even in this form, it should be addressed somewhere in the 5.0 timeframe.

### Pull Request Readiness Checklist

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

- [x] I agree to contribute to the project under Apache 2 License.
- [x] 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
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/gapi/src/executor/gstreamingexecutor.cpp

@@ -41,6 +41,10 @@ using namespace cv::gimpl::stream;
 class VideoEmitter final: public cv::gimpl::GIslandEmitter {
     cv::gapi::wip::IStreamSource::Ptr src;
 
+    virtual void halt() override {
+        src->halt();
+    }
+
     virtual bool pull(cv::GRunArg &arg) override {
         // FIXME: probably we can maintain a pool of (then) pre-allocated
         // buffers to avoid runtime allocations.
@@ -62,6 +66,10 @@ public:
 class ConstEmitter final: public cv::gimpl::GIslandEmitter {
     cv::GRunArg m_arg;
 
+    virtual void halt() override {
+        // Not used here, but in fact can be used.
+    }
+
     virtual bool pull(cv::GRunArg &arg) override {
         arg = const_cast<const cv::GRunArg&>(m_arg); // FIXME: variant workaround
         return true;
@@ -1918,6 +1926,11 @@ void cv::gimpl::GStreamingExecutor::stop()
     for (auto &q : m_emitter_queues) {
         q.push(stream::Cmd{stream::Stop{}});
     }
+    // Also kindly ask emitter object to halt to break the blocking src->pull()
+    // loop
+    for (auto &nh : m_emitters) {
+        m_gim.metadata(nh).get<Emitter>().object->halt();
+    }
 
     // Pull messages from the final queue to ensure completion
     Cmd cmd;