networking: Add optional version check for hosts/clients

[thomasboyt]

web games have the potential for a lot of bugs around weird caching issues and out of date clients, so it'd be good to ensure clients and hosts have the same version

this could be a param added to either the connect() fn or the Networking component params:

new NetworkingHost({
  prefabs,
  maxClients: 2,
  version: "1"
});

implementation of the "version" should be left up to the games, but could be a SHA stamp from the git repo or something, or maybe webpack could handle hashing the bundle?

as for the actual check, client should send it on initial connect and server should send back a new VersionMismatchError message