Support Binding to IReactiveObject's

[worldbeater]

What kind of change does this PR introduce?

Currently, ReactiveUI.Validation doesn't support creating ValidationRules for objects that don't inherit from a ReactiveObject. Also, binding to IViewFor<IReactiveObject> via BindValidation isn't supported. However, there may be cases, when one needs to use an interface as a type parameter in IViewFor (e.g. IViewFor<IViewModel>). Also, some folks may decide to use a custom IReactiveObject implementation e.g. in case if they use ReactiveUI with MvvmCross, Prism, etc. So this PR changes the generic type constraint from : ReactiveObject to : class, IReactiveObject and adds a few tests for this.

What is the current behavior?

Currently, ReactiveUI.Validation doesn't support binding to IReactiveObjects.

What is the new behavior?

Now, ReactiveUI.Validation supports binding to IReactiveObjects.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #114 into main will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   52.51%   52.73%   +0.22%     
==========================================
  Files          16       16              
  Lines         895      895              
==========================================
+ Hits          470      472       +2     
+ Misses        425      423       -2     

Impacted Files	Coverage Δ
Helpers/ReactiveValidationObject.cs	96.96% <0.00%> (+6.06%)	⬆️

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