[FEATURE] Allow environment variables in site config

To enable easy configuration of environment specific
site setting, usage of environment variables in site
configuration yaml has been introduced.

The TYPO3 Core Yaml loader is now able to resolve
variables from environment variables.

Resolves: #86409
Releases: master
Change-Id: Ic1e32d231aa7e92b3feb4ed4c31bed72520d71fb
Reviewed-on: https://review.typo3.org/58358


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>