Scripting interface to implement muxers

[emarsden]

It would be useful to implement a scripting interface to the muxing functionality required by this library. This would make it easier to implement a new muxer, and to change the arguments used for a muxing application (in particular, ffmpeg has many commandline arguments that control encoding which can be useful to change, for example depending on the container format).

We need muxing for:

• muxing an audio and a video stream together
• adding an audio stream to a specified container format
• adding a video stream to a specified container format
• concatenating multiple media containers for multi-Period manifests

This could be implemented using the platform's commandline interface, or a scripting platform such as rhai or Python-in-WASM.