Draft: hlsfmp4sink: init

This change largely extracts the implementation of the previous hlscmafsink into a common base class and exposes an additional variant: hlsfmp4sink which generates arbitrary isofmp4 muxed fragments rather than cmaf ones.

Notable differences currently are just between how the sink pads are handled (fmp4 has requestable pads instead of static ones.)

I tested this new variant by essentially modifying the cmaf_live example with hlsfmp4sink element instead and checked that it does indeed produce ISO shaped mp4s instead of cmaf ones and that the result plays in mpv.

Still, I imagine it would be great to modify the cmaf_live example such that it works in either mode without needing manual modifications to the source (e.g. by adding an argument perhaps?)