Added support for configurable keyframe interval

labs/bbb-webrtc-sfu/config/default.example.yml

.../bbb-webrtc-sfu/lib/mcs-core/lib/media/MediaController.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/media/media-server.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/model/SdpSession.js

labs/bbb-webrtc-sfu/lib/mcs-core/lib/model/SfuUser.js

labs/bbb-webrtc-sfu/lib/screenshare/screenshare.js

labs/bbb-webrtc-sfu/package-lock.json

labs/bbb-webrtc-sfu/package.json