IndexNow: Render key request on `parse_request` hook to improve performance.

[peterwilsoncc]

Currently the FAIR plugin renders the IndexNow key on the template_redirect hook.

Moving the rendering of the file to the parse_request hook will save about 30 database queries (based on my local testing) as WP will prepare to render the front page between parse_request and template_redirect.

Additionally, the official Bing plugin includes the header X-Robots-Tag: noindex for the file to prevent it ending up in search engines, while modifying the rendering of the file, it may be worth adding that to the headers.

I'm happy to do a PR for this if y'all are happy to make one or both of the changes.