Replaced caffe model with onnx for colorization sample

[gursimarsingh]

25006

Improved sample for colorization with onnx model in cpp and python. Added a demo image in data folder for testing

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
• [ ] 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

[LaurentBerger]

https://github.com/opencv/opencv/blob/448375d1e7b9a005b9a7e2c0c74649b54a8c36a1/samples/dnn/action_recognition.py#L48 https://github.com/opencv/opencv/blob/448375d1e7b9a005b9a7e2c0c74649b54a8c36a1/samples/dnn/classification.cpp#L105 https://github.com/opencv/opencv/blob/448375d1e7b9a005b9a7e2c0c74649b54a8c36a1/samples/dnn/edge_detection.py#L46 https://github.com/opencv/opencv/blob/448375d1e7b9a005b9a7e2c0c74649b54a8c36a1/samples/dnn/fast_neural_style.py#L16 https://github.com/opencv/opencv/blob/448375d1e7b9a005b9a7e2c0c74649b54a8c36a1/samples/dnn/object_detection.cpp#L161