[Design] Add `InputTagHelper`

- first part of #1245 (mostly needs tests)
this call is analogous to how `HtmlHelper<TModel>.CheckBoxFor()` calls the `IHtmlGenerator`.
1. the `IHtmlGenerator` method figures out the "checked" attribute value based on the expression result.
2. once I fix handling other attributes on the element in the Razor source, users will be able to type this attribute in (analogous to including "checked" in `htmlAttributes`).