Merge pull request #20367 from augustinmanecy:features2d-rw

**Merge with contrib**: https://github.com/opencv/opencv_contrib/pull/3003

### 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 other license that is incompatible with OpenCV
- [x] The PR is proposed to proper branch
- [ ] There is reference to 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.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/features2d/src/agast.cpp

@@ -7948,6 +7948,27 @@ public:
     : threshold(_threshold), nonmaxSuppression(_nonmaxSuppression), type(_type)
     {}
 
+    void read( const FileNode& fn) CV_OVERRIDE
+    {
+        // if node is empty, keep previous value
+        if (!fn["threshold"].empty())
+            fn["threshold"] >> threshold;
+        if (!fn["nonmaxSuppression"].empty())
+            fn["nonmaxSuppression"] >> nonmaxSuppression;
+        if (!fn["type"].empty())
+            fn["type"] >> type;
+    }
+    void write( FileStorage& fs) const CV_OVERRIDE
+    {
+        if(fs.isOpened())
+        {
+            fs << "name" << getDefaultName();
+            fs << "threshold" << threshold;
+            fs << "nonmaxSuppression" << nonmaxSuppression;
+            fs << "type" << type;
+        }
+    }
+
     void detect( InputArray _image, std::vector<KeyPoint>& keypoints, InputArray _mask ) CV_OVERRIDE
     {
         CV_INSTRUMENT_REGION();