Fix for does not trigger WhenActivated() (#3808)

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**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Fix for #3413

**What is the current behavior?**
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IActivatableViewModel does not activate

**What is the new behavior?**
<!-- If this is a feature change -->

adds ability to hook to IActivatableViewModel Activate and Deactivate

**What might this PR break?**

None expected

**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

src/ReactiveUI.Blazor/ReactiveInjectableComponentBase.cs

@@ -70,6 +70,12 @@ public void Dispose()
     protected override void OnInitialized()
     {
         _initSubject.OnNext(Unit.Default);
+        if (ViewModel is IActivatableViewModel avm)
+        {
+            Activated.Subscribe(_ => avm.Activator.Activate()).DisposeWith(_compositeDisposable);
+            Deactivated.Subscribe(_ => avm.Activator.Deactivate());
+        }
+
         base.OnInitialized();
     }
 
@@ -132,4 +138,4 @@ protected virtual void Dispose(bool disposing)
             _disposedValue = true;
         }
     }
-}
+}