Issue #3605422 by hydra: Add site cache tags to config override metadata

Pascal Crottrequested to merge
issue/sites-3605422:3524112-config-override-cache-tags into 1.x

SiteConfigOverrider::getCacheableMetadata() varied overridden config by the 'site' cache context but added no cache tags, so changes to a site (e.g. saving the entity that provides its overrides) did not invalidate caches built from the overridden config.

It now adds the current site's cache tags ($currentSite->getCacheTags()) for overridden config names. A site plugin that derives its overrides from an entity (e.g. sites_group's GroupSitesBase exposing group:GID) thereby makes saving that entity invalidate the dependent caches.

Closes #3524112

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