Merge pull request #27841 from dkurt:ffmpeg_camera

Open camera device by index through FFmpeg #27841

### Pull Request Readiness Checklist

resolves https://github.com/opencv/opencv/issues/26812

Example of explicit backend option (similar to `-f v4l2` from command line)
```
export OPENCV_FFMPEG_CAPTURE_OPTIONS="f;v4l2"
```
see https://trac.ffmpeg.org/wiki/Capture/Webcam for available options

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
- [x] 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/videoio/test/test_camera.cpp

@@ -327,4 +327,18 @@ TEST(DISABLED_videoio_camera, waitAny_V4L)
     }
 }
 
+TEST(DISABLED_videoio_camera, ffmpeg_index)
+{
+    int idx = (int)utils::getConfigurationParameterSizeT("OPENCV_TEST_FFMPEG_DEVICE_IDX", (size_t)-1);
+    if (idx == -1)
+    {
+        throw SkipTestException("OPENCV_TEST_FFMPEG_DEVICE_IDX is not set");
+    }
+    VideoCapture cap;
+    ASSERT_TRUE(cap.open(idx, CAP_FFMPEG));
+    Mat frame;
+    ASSERT_TRUE(cap.read(frame));
+    ASSERT_FALSE(frame.empty());
+}
+
 }} // namespace