FEATURE: Allow /u/by-external to work for all managed authenticators (#11168)

Previously, `/u/by-external/{id}` would only work for 'Discourse SSO' systems. This commit adds a new 'provider' parameter to the URL: `/u/by-external/{provider}/{id}`

This is compatible with all auth methods which have migrated to the 'ManagedAuthenticator' pattern. That includes all core providers, and also popular plugins such as discourse-oauth2-basic and discourse-openid-connect.

The new route is admin-only, since some authenticators use sensitive information like email addresses as the external id.

lib/auth/managed_authenticator.rb

@@ -1,6 +1,12 @@
 # frozen_string_literal: true
 
 class Auth::ManagedAuthenticator < Auth::Authenticator
+  def is_managed?
+    # Tells core that it can safely assume this authenticator
+    # uses UserAssociatedAccount
+    true
+  end
+
   def description_for_user(user)
     associated_account = UserAssociatedAccount.find_by(provider_name: name, user_id: user.id)
     return "" if associated_account.nil?