imgcodecs: OpenEXR multispectral read:write support (#27485)

imgcodecs: OpenEXR multispectral read/write support #27485

OpenCV Extra: https://github.com/opencv/opencv_extra/pull/1262/

Adds capability to read and write multispectral (>4 channels) images in OpenEXR format.

### 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
- [ ] 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.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgcodecs/include/opencv2/imgcodecs.hpp

@@ -484,7 +484,7 @@ filename extension (see cv::imread for the list of extensions). In general, only
 single-channel or 3-channel (with 'BGR' channel order) images
 can be saved using this function, with these exceptions:
 
-- With OpenEXR encoder, only 32-bit float (CV_32F) images can be saved.
+- With OpenEXR encoder, only 32-bit float (CV_32F) images can be saved. More than 4 channels can be saved. (imread can load it then.)
   - 8-bit unsigned (CV_8U) images are not supported.
 - With Radiance HDR encoder, non 64-bit float (CV_64F) images can be saved.
   - All images will be converted to 32-bit float (CV_32F).