FEATURE: API to create user's associated account (#15737)

Discourse users and associated accounts are created or updated when a
user logins or connects the account using their account preferences.
This new API can be used to create associated accounts and users too,
if necessary.

spec/lib/discourse_spec.rb

@@ -141,7 +141,7 @@ describe Discourse do
             'pluginauth'
           end
 
-          def enabled
+          def enabled?
             true
           end
         end