Add option to reencode stream before Prebuffer
**Is your feature request related to a problem? Please describe.**
Hello, I have an (old) mjpeg Camera added to Homebridge-camera-ui v5.0.0-beta.55. All work well, except that I can't use the 'Prebuffer' option, as the stream is not H264 : 
`[CameraUI] Porche: Can not start Prebuffer, camera does not support H264 stream (mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 640x480, 25 tbr, 25 tbn, 25 tbc)`

As the camera is pretty old, the option would be useful to open only one stream to it, instead of opening multiple that will slow down the camera (1 for motion analysis, 1 for HSV, etc....)

**Describe the solution you'd like**
Could you add an option to reencode the stream to H264 in the prebuffer process, instead of using the 'copy' codec ? That would be an optional parameter, because it's working well for cameras that natively support h264.

**Describe alternatives you've considered**
I tried to use the 'restreamer' project to reencode the stream in H264/RTMP before serving it to Camera-UI. It works, but add a long unnecessary delay.
