webrtc: add features for specific signallers
When swapping between several development branches, compilation times can be frustrating. This commit proposes adding features to control which signaller to include when building the webrtc plugin. By default, all signallers are included, just like before.
Compiling the webrtc-precise-sync
examples with --no-default-features
reduces compilation to 267 crates instead of 429 when all signallers are
compiled in.