- #5028
- helpers similar to our HTML or tag helpers can use the new singleton to examine or add validation attributes
- in the most common case, helpers add validation attributes to a `TagBuilder`
- separate `DefaultValidationHtmlAttributeProvider` from `DefaultHtmlGenerator`
- avoids creating two instances of the `DefaultHtmlGenerator` singleton
- would be even uglier to require callers to cast an `IHtmlGenerator` to `ValidationHtmlAttributeProvider`
- `[Obsolete]` old `DefaultHtmlGenerator` constructor
