Add lifecycle methods to ObservableRecipient

[alejandroortega08]

It would be very useful if we included a powerful set of lifecycle methods. It would help track the state of the corresponding view and it can provide helper methods for more common functionality.

A good example would be a method called "Initialize" that is called after the native navigation has take place. It returns a Task, so can safely be marked async and used for heavy loading operations (such as hitting a backend API), without blocking the UI thread.

https://www.mvvmcross.com/documentation/fundamentals/viewmodel-lifecycle