[SDK] rpc subsystem updates

[eanders-ms]

Changes to the RPC subsystem to introduce the notion of "channels". An app can now register an RPC dispatcher to a channel, and messages sent on the channel will be passed to the corresponding dispatcher, which will in turn find and call the correct handler. Example:

// In App constructor:
  // Create an rpc dispatcher.
  const rpc = new RPC(context);
  // Register the rpc dispatcher to the "video-player" channel.
  context.rpcChannels.setChannelHandler(rpc, "video-player");
  // Add a handler to the rpc dispatcher. Will handle the "play-video" message sent on the "video-player" channel.
  rpc.on("play-video", (url) => this.playVideo(url));

This design allows for better separation of concerns particularly on the C# side. Using channel-based rpc is optional.

Also in this PR:

• Added userId parameter to the rpc handler invocation.
• Removed the notion of UserRPC vs. ContextRPC. All rpc happens on context, and an optional userId can be passed to filter the call to a single user. Otherwise it is sent to all users.
• Fixed a bug where rpc parameters were being passed as an array instead of destructured.
• Removed altspacevr-extras package.

Why this change?
For the Web Projector project (the remote web browser), I needed the id of the user making the rpc call on the Node side so that I could check whether they're authorized to do what they're trying to do. At the same time, I was pondering how to organize the code on the C# side in a nicely separated way. The introduction of channels allows for better code structure, and more succinct rpc procedure naming.

[tombuMS]