feat: gst compositor - work in progress

[leewardbound]

thanks to @billylindeman and ion-cluster for the code!

• expanded existing gst example using ion-cluster examples

• now we should support both appsrc and appsink

• example save-to-disk with compositor is not fully working yet (saves AVIs with expected filesize, but unplayable, maybe needs an ending terminator?)

[leewardbound]

instructions for testing:

• launch ion-sfu allrpc -- go run cmd/signal/allrpc/main.go -jaddr :7000 -gaddr :50051

• modify ion-sfu/examples/pubsubtest/main.js -- add codec: "h264" on line 63

• start the project - go run main.go -session "test session"

• open pubsubtest and attach a video feed

• open pubsubtest in a second tab and attach a video feed again

• close the tabs to stop the pipeline