Add async overload of SubscribeToAction

[pcannon-b2w]

When using FluxorComponent.SubscribeToAction to work with mutable objects (such as DTO's), I often need to transition into async code. An example would be where we are binding a DataGrid to a DTO. The 3rd party components methods for refreshing are asynchronous.

In this scenario, it would really useful to have an async variant for the SubscribeToAction method, replacing the Action<TAction> callback parameter with Func<TAction, Task> callback and then awaiting this inside of InvokeAsync. This avoids StateHasChanged being called before the handler has fully completed.

In my own project, I have a custom version of FluxorComponent where we have implemented this and it is working well. I wanted to suggest adding this to the base component.