Expose APIs for clearing the ECMs internal state tracking addition and removal (#3002)

This is necessary for use with the `SetState` API. This API is commonly
used to create a local version of the servers ECM that is periodically
synchronized. However, without exposing this function into our public API
additions and removals of entities or components are not reflected in the
local ECM.

Currently, we use `friend` relationships for the `GuiRunner` class to
allow it to run these functions, but this is not feasible for external
code that wants to use `SetState`.

https://github.com/gazebosim/gz-sim/blob/5ce62a8826b930078fae940288ac9677aae9ec7b/include/gz/sim/EntityComponentManager.hh#L834,
https://github.com/gazebosim/gz-sim/blob/5ce62a8826b930078fae940288ac9677aae9ec7b/src/gui/GuiRunner.cc#L316-L318

Signed-off-by: Addisu Z. Taddese <[email protected]>
(cherry picked from commit b2f3c82)

Author: azeey

include/gz/sim/EntityComponentManager.hh

@@ -684,21 +684,19 @@ namespace gz
                   const std::string &_name) const;
 
       /// \brief Clear the list of newly added entities so that a call to
-      /// EachAdded after this will have no entities to iterate. This function
-      /// is protected to facilitate testing.
-      protected: void ClearNewlyCreatedEntities();
+      /// EachAdded after this will have no entities to iterate.
+      public: void ClearNewlyCreatedEntities();
 
       /// \brief Clear the list of removed components so that a call to
       /// RemoveComponent doesn't make the list grow indefinitely.
-      protected: void ClearRemovedComponents();
+      public: void ClearRemovedComponents();
 
       /// \brief Process all entity remove requests. This will remove
-      /// entities and their components. This function is protected to
-      /// facilitate testing.
-      protected: void ProcessRemoveEntityRequests();
+      /// entities and their components.
+      public: void ProcessRemoveEntityRequests();
 
       /// \brief Mark all components as not changed.
-      protected: void SetAllComponentsUnchanged();
+      public: void SetAllComponentsUnchanged();
 
       /// Compute the diff between this EntityComponentManager and _other at the
       /// entity level. This does not compute the diff between components of an