Generally we want people to use the existing plugins to manage their
authentication, however there are a number of existing services that
know how to work with an AccessInfo object directly and either cache it
or manipulate it manually.
Provide a simple Identity plugin that just takes an existing AccessInfo
and allows it to be used as an authentication plugin.
Change-Id: I388283c03a0a8a3d1afe43138eebbe5e66ca9102
59cdbe8ed4