Separate Login services for admin/front-end

[sebastienros]

c.f. the discussion during this meeting: https://youtu.be/dyFNRnd8dMo?t=3034

The idea is that sharing the same APIs for front-end users and back-end ones might be an issue. The best example is an e-commerce where the site's admins should not be mixed with users. And maybe the stores should be also different, some being put in common. Or maybe the external login providers could be different.

Right now the default behavior is to have a single store and set of APIs. A solution would be to have an optional feature and configuration dedicated for the front-end.