Add Keyframe Caching (prepush frame)

[thinkjk]

Request to add keyframe caching from this conversation: https://github.com/AlexxIT/go2rtc/issues/35#issuecomment-1299527683

[thinkjk]

Hey @AlexxIT any chance this will be added soon?

[AlexxIT]

There are no plans to do it "soon". There are other features to do. I consider this problem a cosmetic one. On good cameras it does not exist at all. It is a complex problem and requires rewriting the kernel.

[swazrgb]

I'm taking a stab at this here: https://github.com/AlexxIT/go2rtc/compare/master...swazrgb:go2rtc:frame-caching

Unfortunately the diff is quite large due to the dependency of the h264 package on the core package, which required moving quite a lot of stuff to a new common package