Open VideoCapture from memory buffer

[dkurt]

Pull Request Readiness Checklist

Suppoted backends:

• FFmpeg
• MSMF

resolves https://github.com/opencv/opencv/issues/24400

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

[dkurt]

Discussed on OpenCV weekly sync about potential improvements:

1. On current PR state, do not use std::vector but allow user pass just pointer/buffer_size.
2. Consider alternative API with a kind of streaming by chunks. Might be more efficient with GPU decoders.

[mshabunin]

We have something similar in cudacodec module:

RawVideoSource object is an iterator over encoded data stream (https://github.com/opencv/opencv_contrib/blob/b236c71c2f8d983403c35a0cea8bec0432a4b0fe/modules/cudacodec/include/opencv2/cudacodec.hpp#L508-L555) and can be used to construct VideoReader (https://github.com/opencv/opencv_contrib/blob/b236c71c2f8d983403c35a0cea8bec0432a4b0fe/modules/cudacodec/include/opencv2/cudacodec.hpp#L604).

Following example in the test decodes key frames (if I understand corretly): https://github.com/opencv/opencv_contrib/blob/b236c71c2f8d983403c35a0cea8bec0432a4b0fe/modules/cudacodec/test/test_video.cpp#L178-L209