support for Signed video - code update

I'd like to suggest a small bit of code to add support for signed video (https://github.com/AxisCommunications/signed-video-framework)

I've forked the repo and added the bit of code that honestly I just copied and pasted from the source code, IMHO so easy wouldn't be, great code!

https://github.com/zeparedes/media-stream-library-js/tree/add_support_signed_video