rabbit_feature_flags: Improve feature_props_extended() spec

The `provided_by` field is required. This permitted to discover a wrong
spec of `rabbit_ff_registry:get/1`.

While here, sort `feature_props_extended()` fields like
`feature_props()`.

Author: dumbbell

deps/rabbit/src/rabbit_feature_flags.erl

@@ -175,10 +175,10 @@
 -type feature_props_extended() :: #{desc => string(),
                                     doc_url => string(),
                                     stability => stability(),
+                                    depends_on => [feature_name()],
                                     callbacks =>
                                     #{callback_name() => callback_fun_name()},
-                                    depends_on => [feature_name()],
-                                    provided_by => atom()}.
+                                    provided_by := atom()}.
 %% The feature flag properties, once expanded by this module when feature
 %% flags are discovered.
 %%

deps/rabbit/src/rabbit_ff_registry.erl

@@ -63,7 +63,7 @@
         end).
 
 -spec get(rabbit_feature_flags:feature_name()) ->
-    rabbit_feature_flags:feature_props() | undefined.
+    rabbit_feature_flags:feature_props_extended() | undefined.
 %% @doc
 %% Returns the properties of a feature flag.
 %%
@@ -75,7 +75,10 @@
 
 get(FeatureName) ->
     _ = rabbit_ff_registry_factory:initialize_registry(),
-    ?convince_dialyzer(?MODULE:get(FeatureName), undefined, #{}).
+    ?convince_dialyzer(
+       ?MODULE:get(FeatureName),
+       undefined,
+       #{provided_by => rabbit}).
 
 -spec list(all | enabled | disabled) -> rabbit_feature_flags:feature_flags().
 %% @doc