Cascading values+params for state management

[guardrex]

Fixes #34236

Thanks @hakenr! 🎷 ... This is probably a bit rough in spots. If you have time to look, I'll address your feedback before pinging a PU engineer. If you're busy, no worries ... I'll get one of them on here early next week to put an 👁️ on this.

Notes

• Add a CascadingValueSource<T> with notifications (INotifyPropertyChanged/NotifyChangedAsync) approach to the Cascading values and parameters article. The basis for the example is code provided by @kzu on CascadingValueSource<T> should automatically invoke NotifyChangedAsync for INotifyPropertyChanged values aspnetcore#53257 (Thanks @kzu! 🎸) . Sanderson said it's supported at CascadingValueSource<T> should automatically invoke NotifyChangedAsync for INotifyPropertyChanged values aspnetcore#53257 (comment), but he also commented elsewhere that "In practice it's extremely rare for people to want to do INotifyPropertyChanged with Blazor." However, that might be because it isn't documented (until now here 😄).

• I favor placing this coverage because it's easy to understand/implement and flexible/powerful with a class type. I demo both setting a property value directly and having a method in the class that updates a property. An app can process state quite easily using this approach. I make sure to call out that there's a perf hit and to be careful with this approach.

• Add a short section to the State Management article mentioning cascading values and parameters generally. For >=8.0, cross-link the root-level cascading value/CascadingValueSource<T> /INotifyPropertyChanged/NotifyChangedAsync approach.

• WRT ...

  the entire State Management page could simply be a list of approaches with links to other pages that explain each feature in detail

  My initial concern is simply that explaining the features and explaining state management using the features in one spot is problematic because state management is a more advanced use case for the two or three scenarios here where this idea applies. For root-level cascading values with notifications, this approach seems to work well because the use case is a state management approach. I've made a tracking item entry for this idea to look at it again later.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/components/cascading-values-and-parameters.md	aspnetcore/blazor/components/cascading-values-and-parameters
aspnetcore/blazor/state-management.md	aspnetcore/blazor/state-management