[TASK] Provide middleware to append admPanel

[kitzberger]

This adds the admin panel's HTML to the end of PAGE's JSON:

{
   ...,
   "admPanel": "<!-- TYPO3 admin panel start --> ... <!-- TYPO3 admin panel end -->"
}

The idea is that the nuxtjs application can attach this HTML string "as is" right before the closing </body> tag.

Unfortunately there's some characters that cause problems and invalidate the resulting JSON ;-(

I can't find a way to correctly escape those characters. But I tracked it down to this symfony HtmlDumper here: https://github.com/TYPO3/typo3/blob/main/typo3/sysext/adminpanel/Classes/Modules/Debug/Events.php#L77