Expose cudacodec::VideoReader::format() to python bindings

[cudawarped]

Fix issue identified in #3195 by exposing cudacodec::VideoReader::format() to python bindings and updating the python test to check verify this.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• 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.
• The feature is well documented and sample code can be built with the project CMake

force_builders=Custom
buildworker:Custom=linux-4,linux-6
build_image:Custom=ubuntu-cuda:18.04