Some QOL improvements

Not much new in this one, here's an overview:

• musicUserToken is not an authentication method anymore but rather a static property of NetworkRequestHeaderField (.musicUserToken(userToken))
• the method urlRequest() -> URLRequest on NetworkRequest is now private (this wasn't meant to be configurable anyways)
• you can now schedule a request directly by passing a Network instance to it, like this for example:

// ...
request.schedule { result in
    // Handle result
}