feat: G-API: Custom stream sources in Python (#27276)

Implemented:

- A C++ proxy class PythonCustomStreamSource that implements the
  IStreamSource interface. This class acts as a bridge between
  G-API’s internal streaming engine and user-defined Python
  objects. Internally, it stores a reference to a Python object
  (PyObject*) and is responsible for: calling the Python object’s
  pull() method to retrieve the next frame, calling the descr_of()
  method to obtain the frame format description, acquiring and
  releasing the Python GIL as needed, converting the returned
  numpy.ndarray into cv::Mat, and handling any exceptions or
  conversion errors with proper diagnostics.

- A Python-facing factory function, cv.gapi.wip.make_py_src(),
  which takes a Python object as an argument and wraps it into a
  cv::Ptr<IStreamSource>. Internally, this function constructs a
  PythonCustomStreamSource instance and passes the Python object to
  it. This design allows Python users to define any class that
  implements two methods: pull() and descr_of(). No subclassing or
  special decorators are required on the Python side. The user
  simply needs to implement the expected interface.

Co-authored-by: Leonor Francisco <leonor.francisco@tecnico.ulisboa.pt>

modules/gapi/misc/python/test/test_gapi_streaming.py

@@ -539,7 +539,38 @@ try:
                 self.assertEqual(0.0, cv.norm(convertNV12p2BGR(expected1), actual1, cv.NORM_INF))
                 self.assertEqual(0.0, cv.norm(convertNV12p2BGR(expected2), actual2, cv.NORM_INF))
 
+        def test_python_custom_stream_source(self):
+            class MySource:
+                def __init__(self):
+                    self.count = 0
 
+                def pull(self):
+                    if self.count >= 3:
+                        return None
+                    self.count += 1
+                    return np.ones((10, 10, 3), np.uint8) * self.count
+
+                def descr_of(self):
+                    return np.zeros((10, 10, 3), np.uint8)
+
+            g_in = cv.GMat()
+            g_out = cv.gapi.copy(g_in)
+            c = cv.GComputation(g_in, g_out)
+
+            comp = c.compileStreaming()
+
+            src = cv.gapi.wip.make_py_src(MySource())
+            comp.setSource([src])
+            comp.start()
+
+            frames = []
+            while True:
+                has_frame, frame = comp.pull()
+                if not has_frame:
+                    break
+                frames.append(frame)
+
+            self.assertEqual(len(frames), 3)
 
 except unittest.SkipTest as e: