[FEATURE] Add frontend debug output as HTTP Response Header

The current debug information in the frontend is added as HTML
comment, but breaks non-HTML pages (e.g. JSON output).

The debug information (= parse time of the frontend request) is now sent as HTTP
Response Header found at "X-TYPO3-Parsetime".

Resolves: #82419
Releases: master
Change-Id: Ibd84825fd3ebdcb1c88e0059d00d78b4bb20f53e
Reviewed-on: https://review.typo3.org/54075


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Hölzle <typo3@markus-hoelzle.de>
Tested-by: Markus Hölzle <typo3@markus-hoelzle.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>