Modify HtmlRenderer to update content when a component re-renders

[pranavkm]

Prior to this change HtmlRenderer would return a snapshot of a component immediately after it was initially rendered. This meant any updates to a component as a result of other components rendering were lost.

In this change, we update the HtmlRenderer to return a placeholder IHtmlContent that retrieves render frames only when WriteTo is invoked. Typically MVC will defer calling WriteTo until the content is about to be sent to the wire, which means any updates to a component (e.g. HeadOutlet being updated) are picked up.

Also updates the template to consolidate _Host and _Layout.

Fixes #38400

[pranavkm]

Need to test this with more cases (and in particular with whatever magical thing we're doing with select elements), but this worked for the Blazor Server sample.

[SteveSandersonMS]

This is

Very nice! Especially with the removal of OnComponentRendered, this now looks so simple and natural. I guess it was always meant to be this way.

I'd approve this PR, but it's still in draft.

[SteveSandersonMS]

Approving even though it's draft

[javiercn]

Yeah, this is much nicer