[Design] Adding support for flush points in Razor pages

Fixes #1042
Part of it was to simply verify if the page executes. I think the right test would be to check if we get the appropriate number of chunks with the contents split for each invocation of Flush. The API that we have on `client` isn't extensive enough to allow this. I an speak to @javiercn to figure out if there's some way to get to the raw results