Added function to set a custom media stream after initialisation

[pat154]

Added setCustomMediaStream function that is returned from the media recorder hook. This allows us to set the custom media stream after initialisation. This can be useful in cases where we want to use custom logic when selecting a media stream and allows us to set the media stream independently from starting a recording.

In my case, I needed to display a countdown after selecting media but before recording started. This function allowed me to do that by manually calling getDisplayMedia and passing the value into setCustomMedia before calling startRecording.