Add Clear-Site-Data and related route for 2.x

Good point, should please be added to 2.x!

I'm not sure if we didn't already implement this in the meantime, but if not, it *might* make sense. @Grevil could you look into the details, please?

This isn't implemented yet and totally makes sense, I'd say!

Although I am not sure yet, if we want to send the header, if the user logs in / out. As the user might still want the cached data even if logged out. Eventually we could send the header somehow, if the user looses the 'access pwa' permission?

What are your thoughts on this @Anybody?

First implementation.

We still need to think, when to execute the clearing, as stated above, and what cache tags / contetxt to assign.

I think when implementing this for login and logout, we should move that functionality into a service that can be reused.
Furthermore, I think this should be configurable for these two cases?

Will need some more work. Happy to review patches, but the maintainers currently don't plan putting further efforts into this. Community members are requested to help.