FPS Adjustment Methods

Currently we always set a fixed FPS, based on the input specification. Depending on product/customer requirements, we may need to consider supporting various other methods of framerate adjustment:

* ~~Passing through the FPS and encoding each frame as-is (and maybe leaving the corresponding pts timestamps untouched?). This should be a matter of removing the `fps` specification from the filter string if needed.~~ (done in https://github.com/livepeer/go-livepeer/issues/1239 )
* Low-pass or high-pass fps adjustment. This may require writing a new filter (or extending the current fps filter).
* Speeding up or slowing down the video via "stretching" or "squeezing" the pts with the setpts filter. This pts adjustment is different from the fps filter's method of duplicating/dropping whole frames.

Reference: Apple HLS Authoring Spec https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices