Implement fp16 support in the new DNN engine

System Information

Platform: any Reference: https://github.com/opencv/opencv/pull/26056

Detailed description

Steps to reproduce

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)