Support Artifact Binding for mobile use cases

[jsmarr]

1. Artifact Binding

• See Nat’s AB Profile for OAuth2
• AB Responses
  • OpenID Connect responses are much shorter than OpenID 2.0 responses
  • AB Profile for OAuth2 will automatically enable AB for OIC
• AB Request
  • Request Artifact is in Nat's AB proposal, but not currently in OIC, should be addressed with the OAuth2 AB Profile.

[davidrecordon]

Given Nat's work already, don't think this is that hard. Is definitely more on the "nice to have" side for me but don't see putting it in as being very contentious.