Introduce a component for initializing the XWikiContext

Currently, the XWikiContext is initialized in different parts of the platform. It's the case in the old core, but also in the WYSIWYG module that has its own XWikiContextInitializationFilter.

As we are currently facing the problem of initializing the XWikiContext in the case of ResourceReferenceHandlers (see XWIKI-14982), it would be interesting to introduce a component that does the work of initializing the XWikiContext and that we could reuse in all the parts of the platform.