Implement RFC 8414 for OAuth 2.0 server metadata (#29191)

config/routes.rb

@@ -62,6 +62,7 @@ Rails.application.routes.draw do
                 tokens: 'oauth/tokens'
   end
 
+  get '.well-known/oauth-authorization-server', to: 'well_known/oauth_metadata#show', as: :oauth_metadata, defaults: { format: 'json' }
   get '.well-known/host-meta', to: 'well_known/host_meta#show', as: :host_meta, defaults: { format: 'xml' }
   get '.well-known/nodeinfo', to: 'well_known/node_info#index', as: :nodeinfo, defaults: { format: 'json' }
   get '.well-known/webfinger', to: 'well_known/webfinger#show', as: :webfinger