Rabbit registry: Dinamically allow to define new classes

rabbit_registry classes are currently static, hard-coded in rabbit registry.
Dinamically searching for new classes using Erlang attributes allows
for plugins to add new classes and make use of the registry.
i.e. shovel protocols can benefit of this feature

(cherry picked from commit ca2bdfb)

Author: dcorbacho

deps/rabbit_common/src/rabbit_registry.erl

@@ -190,7 +190,18 @@ class_module(policy_validator)    -> rabbit_policy_validator;
 class_module(operator_policy_validator) -> rabbit_policy_validator;
 class_module(policy_merge_strategy)     -> rabbit_policy_merge_strategy;
 class_module(ha_mode)                   -> rabbit_mirror_queue_mode;
-class_module(channel_interceptor)       -> rabbit_channel_interceptor.
+class_module(channel_interceptor)       -> rabbit_channel_interceptor;
+class_module(Other) ->
+    AttrsPerApp = rabbit_misc:rabbitmq_related_module_attributes(rabbit_registry_class),
+    find_class_module(Other, AttrsPerApp).
+
+find_class_module(Class, [{_, Module, List} | Rest]) ->
+    case lists:member(Class, List) of
+        true -> Module;
+        false -> find_class_module(Class, Rest)
+    end;
+find_class_module(Class, []) ->
+    throw({invalid_class, Class}).
 
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