Merge pull request #24961 from savuor:rv/ply_mesh

PLY mesh support #24961

**Warning:** The PR changes exising API.

Fixes #24960
Connected PR: [#1145@extra](https://github.com/opencv/opencv_extra/pull/1145)

### Changes
* Adds faces loading from and saving to PLY files
* Fixes incorrect PLY loading (see issue)
* Adds per-vertex color loading / saving

### 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
- [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

samples/python/point_cloud.py

@@ -15,7 +15,7 @@ cv.viz3d.setGridVisible("window", True)
 
 cv.waitKey(0)
 
-vertices, _, indices = cv.loadMesh("../data/teapot.obj")
+vertices, indices = cv.loadMesh("../data/teapot.obj")
 vertices = np.squeeze(vertices, axis=1)
 
 cv.viz3d.showMesh("window", "mesh", vertices, indices)