Materials: Model Manager External Interface (#20825)

* Materials: Model Manager External Interface

Implement the ModelManagerExternal class for the external Materials
interface.

This is part of the ongoing merges of code to support external material
interfaces. In this PR the ModelManagerExternal class is implemented,
along with changes to supporting classes required for this class.

* Apply reviewer feedback

src/Mod/Material/App/ExternalManager.cpp

@@ -116,7 +116,7 @@ void ExternalManager::instantiate()
 
         Py::Callable managerClass(mod.getAttr(_className));
         _managerObject = managerClass.apply();
-        if (_managerObject.hasAttr("APIVersion")) {
+        if (!_managerObject.isNull() && _managerObject.hasAttr("APIVersion")) {
             _instantiated = true;
         }