Feature: #87380 - Introduce SiteLanguageAwareInterface to denote site language awareness 

See forge#87380

Description 

A SiteLanguageAwareInterface with the methods setSiteLanguage(Entity\SiteLanguage $siteLanguage) and getSiteLanguage() has been introduced. The interface can be used to denote a class as aware of the site language.

Impact 

Routing aspects respecting the site language are now using the SiteLanguageAwareInterface in addition to the SiteLanguageAwareTrait. The AspectFactory check has been adjusted to check for the interface _or_ the trait. If you are currently using the trait, you should implement the interface as well.