rabbit_feature_flags: Use non-blocking call in get_state/1

[Why]
The previous implementation was using the blocking `is_enabled/1` API.
This meant that if a feature flag was being enabled and the enable
callback took time, the CLI's `list_feature_flag` command or any use of
the management UI would block until the feature flag was enabled.

[How]
`get_state/1` now uses the non-blocking API. However it returns a now
possible value: `state_changing`.

Author: dumbbell

deps/rabbit/src/rabbit_feature_flags.erl

@@ -719,30 +719,38 @@ info() ->
 info(Options) when is_map(Options) ->
     rabbit_ff_extra:info(Options).
 
--spec get_state(feature_name()) -> enabled | disabled | unavailable.
+-spec get_state(feature_name()) -> enabled |
+                                   state_changing |
+                                   disabled |
+                                   unavailable.
 %% @doc
 %% Returns the state of a feature flag.
 %%
 %% The possible states are:
 %% <ul>
 %% <li>`enabled': the feature flag is enabled.</li>
+%% <li>`state_changing': the feature flag is being enabled.</li>
 %% <li>`disabled': the feature flag is supported by all nodes in the
 %%   cluster but currently disabled.</li>
 %% <li>`unavailable': the feature flag is unsupported by at least one
 %%   node in the cluster and can not be enabled for now.</li>
 %% </ul>
 %%
 %% @param FeatureName The name of the feature flag to check.
-%% @returns `enabled', `disabled' or `unavailable'.
+%% @returns `enabled', `state_changing', `disabled' or `unavailable'.
 
 get_state(FeatureName) when is_atom(FeatureName) ->
-    IsEnabled = is_enabled(FeatureName),
+    IsEnabled = is_enabled(FeatureName, non_blocking),
     case IsEnabled of
-        true  -> enabled;
-        false -> case is_supported(FeatureName) of
-                     true  -> disabled;
-                     false -> unavailable
-                 end
+        true  ->
+            enabled;
+        state_changing ->
+            state_changing;
+        false ->
+            case is_supported(FeatureName) of
+                true  -> disabled;
+                false -> unavailable
+            end
     end.
 
 -spec get_stability

deps/rabbit/src/rabbit_ff_extra.erl

@@ -174,6 +174,8 @@ info(FeatureFlags, Options) ->
                      {State, Color} = case State0 of
                                           enabled ->
                                               {"Enabled", Green};
+                                          state_changing ->
+                                              {"(Changing)", Yellow};
                                           disabled ->
                                               {"Disabled", Yellow};
                                           unavailable ->