openid: discovery endpoint

[mgenov]

OAuth2 library need to serve a discovery endpoint which to describes the endpoint locations:

{
 "issuer": "https://accounts.google.com",
 "authorization_endpoint": "https://accounts.appdomain.com/o/oauth2/v2/auth",
 "token_endpoint": "https://www.appdomain.com/oauth2/v4/token",
 "userinfo_endpoint": "https://www.appdomain.com/oauth2/v3/userinfo",
 "revocation_endpoint": "https://accounts.appdomain.com/o/oauth2/revoke",
 "jwks_uri": "https://appdomain.com/oauth2/v3/certs",
 "response_types_supported": [
  "code",
  "token",
  "id_token",
  "code token",
  "code id_token",
  "token id_token",
  "code token id_token",
  "none"
 ],
 "subject_types_supported": [
  "public"
 ],
 "id_token_signing_alg_values_supported": [
  "RS256"
 ],
 "scopes_supported": [
  "openid",
  "email",
  "profile"
 ],
 "token_endpoint_auth_methods_supported": [
  "client_secret_post",
  "client_secret_basic"
 ],
 "claims_supported": [
  "aud",
  "email",
  "email_verified",
  "exp",
  "family_name",
  "given_name",
  "iat",
  "iss",
  "locale",
  "name",
  "picture",
  "sub"
 ],
 "code_challenge_methods_supported": [
  "plain",
  "S256"
 ]
}