Add userinfo oauth endpoint (#32548)

config/routes.rb

@@ -64,6 +64,13 @@ Rails.application.routes.draw do
                 tokens: 'oauth/tokens'
   end
 
+  namespace :oauth do
+    # As this is borrowed from OpenID, the specification says we must also support
+    # POST for the userinfo endpoint:
+    # https://openid.net/specs/openid-connect-core-1_0.html#UserInfo
+    match 'userinfo', via: [:get, :post], to: 'userinfo#show', defaults: { format: 'json' }
+  end
+
   scope path: '.well-known' do
     scope module: :well_known do
       get 'oauth-authorization-server', to: 'oauth_metadata#show', as: :oauth_metadata, defaults: { format: 'json' }