Feature: #99047 - Load site settings from separate settings.yaml

See forge#99047

Description

Site settings have been introduced with TYPO3 v10 as part of the configuration of a site. In contrast to the site configuration, they are mostly used to provide sane defaults for TypoScript constants and have a layer of arbitrary configuration available in any context.

In order to separate these settings from the system site configuration, make them accessible and editable in the TYPO3 backend, and to distinguish between required site configuration and optional settings, the "settings" part of the settings are moved to a separate settings.yaml file in the site configuration folder.

A migration wizard is provided as upgrade wizard to migrate settings into the new file.

Impact

Settings are now loaded from a separate file called settings.yaml residing next to the config.yaml of a site. Executing the upgrade wizard will load all settings of a site and create that file for the user. The migration wizard will not remove / rewrite the config.yaml - the user should do that on their own, to avoid breaking custom-built functionality.