Generic Compiler Pass to replace admin class or document class

[ElectricMaxxx]

I currently had an idea how to replace the editable admin/document class configuration in a nice way.
On my sandbox PR (symfony-cmf/cmf-sandbox#332) i created a very specific compiler pass to replace the document class for the StaticContentAdmin, what about a configuration here like:

admin_classes:
    cmf_sonata_admin_integration.content.admin: My\Special\Admin\Class
    ....
document_classes:
   cmf_sonata_admin_integration.content.admin: AppBundle\Document\DemoSeoContent
   ....

We could then create a more generic compiler pass to replace the class or the 2. argument of a service (key) with the given value.

[ElectricMaxxx]

I think we won't need that on our first release, right @wouterj ? Or why do you add it into the "Sprint 2.0 RC board"?