OpenID Connect 1.0 Client

Implements the Code Flow (OAuth2 Authorization Code Grant) for OpenID
Relying Parties (RP).  Provides a pluggable API using Ring handlers
and Ring middleware.

Flow:
- User Agent requests the authentication endpoint
- Client resolves the OpenID Provider (OP) to use and validates the
  request
- Client runs the :authentication-hook
- Client redirects User Agent to Provider
- Provider authenticates user and redirects the User Agent to the
  callback endpoint
- Client validates the request and prepares to fetch tokens
- Client runs the :authorization-code-hook
- Client requests tokens at the Provider
- Provider responds with tokens
- Client validates the response and the ID token
- Client runs the :token-hook
- Client requests user info at the Provider
- Provider responds with user info
- Client validates the response and the user info
- Clients runs the :userinfo-hook
- Client redirects the User Agent to the completion URI

Hooks:
  :authentication-hook
  :authorization-code-hook
  :token-hook
  :userinfo-hook

Handlers:
  :authentication-response-handler
  :callback-response-handler
  :error-handler

See: https://openid.net/specs/openid-connect-basic-1_0.html