Change the checkbox fork in InputTagHelper to work with other TagHelpers

[NTaylorMullen]

Today the GenerateCheckbox method in our InputTagHelper is inserted into the body of the TagHelperOutput. Functionally this works but it prevents other TagHelpers from acting on the current TagHelperOutput's tagname, attributes etc. We originally built the GenerateCheckbox impl as is because we'd always generate a hidden input tag inline; now, we have access to PostElement AND we have a separate TagHelper that renders the hidden input tag.

[dougbu]

Think this is a dupe of #5902. As I mentioned in response to that bug, need to handle the !ViewContext.FormContext.CanRenderAtEndOfForm case. Guess we could instead find a way to require end of <form> rendering support.