Pac4J OAuth/OpenID Principal Needs to be Configurable

Currently, the Pac4JIdentityAdapter blindly accepts the subject of the returned UserProfile which isn't directly usable in the Hadoop operating environment. We need to be able to resolve it to an actual username.

It seems that we could take two different approaches for this.

1. Add a param to the pac4j provider to indicate the UserProfile attribute to use as the PrimaryPrincipal
2. Add a new identity assertion provider that can decrypt the pac4jUserProfile cookie and extract the configured attribute.

I lean towards #1 above so that identity assertion providers could be used to munge the extracted attribute in interesting ways.

There was some discussion of this [1] back in 0.8.0 and we never really circled back to it.
jleleu - Am I missing anything that is already in place for this?

1. http://mail-archives.apache.org/mod_mbox/knox-dev/201601.mbox/%3CCACRbFyitvZ72-oqu2triGmn%3DKhB8JE0pFONyFim63RKS4gZp0A%40mail.gmail.com%3E