Add nvidia hardware acceleration (beta)

[zoriya]

Todo before merging:

• write install guide
• check if there are issues with max-concurrent encode of nvidia (look https://github.com/keylase/nvidia-patch) added a note on install guide, will check after merging inital pr

Notes for later:

• the whole pipeline is done on gpu, if something should be done on cpu space, we can use the filters hwdownload_cuda and hwupload_cuda to send them to cpu or gpu space respectively
• if we encounter issues with interlaced videos, we might need to deinterlace them (done via the yadif_cuda filter)
• if we encounter decode issues with some coded, the -hwaccel cuda used for decoding might need to be replaced by a -hwaccel auto since it can fallback to cpu decode in case of errors.