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/src/pysrc/python_stream_source.cpp

@@ -0,0 +1,17 @@
+#include <opencv2/gapi/pysrc/python_stream_source.hpp>
+#include <opencv2/gapi/streaming/source.hpp>
+#include <opencv2/core/utils/logger.hpp>
+#include <opencv2/core.hpp>
+
+namespace cv {
+namespace gapi {
+namespace wip {
+
+cv::Ptr<cv::gapi::wip::IStreamSource> make_py_src(const cv::Ptr<cv::gapi::wip::IStreamSource>& src)
+{
+    return src;
+}
+
+} // namespace wip
+} // namespace gapi
+} // namespace cv