Pre/Post Binding Execute
## 🚀 Feature Requests

Ability to trigger an action in the view's fluent binding before or after executing a command.

### Contextualize the feature

It would be useful to be able to trigger an action/function in the view before or after executing a command. For example, I was considering how to use the new MvxPopoverPresentation attribute. This would be a useful place to set the source rect.

### Describe the feature

Here's an example of what it might look like.

```
            var set = CreateBindingSet();
            set.Bind(button)
                .To(vm => vm.ButtonCommand)
                .PreExecute(() =>
                {
                    //Do something just before invoking the command
                })
                .PostExecute(() =>
                {
                    //Do something just after invoking the command
                });
            set.Apply();
```

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android
- [x] :checkered_flag: WPF
- [x] :earth_americas: UWP
- [x] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms
- [x] :🌍: Blazor
