Experimental Vp9 SVC support

[Arri98]

Added VP9 SVC support via SDP munging following Mediasoup and Iñaki Baz Castillo approach, so credits to him. The existing VP9 packet parser included in Licode supports this type of packets so only changes in the client are necessary. It only works with Chrome starting from chrome 74 and is not officially supported. To enable it publish the stream with the following flag:
room.publish(localStream, {svc:true});
In my tests, without using any chrome flag, the browser sends 2 spatial layers (corresponding with licode's layer 0 and 1) and three spatial layers (corresponding with licode's layer 0, 1 and 2)

Also added controls in the basicExample to switch between layers when SVC or simulcast are active.

[] It needs and includes Unit Tests

Changes in Client or Server public APIs

TODO: Write the documentation for the final PR.
[X] It includes documentation for these changes in /doc.