[BUGFIX] Allow protocol-relative urls in AssetRenderer

Allow including protocol-relative urls (urls that start with "//") in
the new AssetRenderer in the same way as it is already supported in
other parts of TYPO3.

Keep in mind, that using protocol-relative urls is considered an
antipattern nowadays and if you run your website solely on encrypted
connections (which you should by now), you can and should always request
all resources via https://.

Resolves: #90933
Releases: master
Change-Id: Iaf91091fca64fde889501f5ac55dbb943d4c116e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64069


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Benni Mack <benni@typo3.org>