Added ability to convert multiply-nested slices (#1239)

* Addressed feedback from previous PR
* Change missing entity aliases to be deterministic.
When converting slices, this helps produce the same results on multiple reconversions of the same data.
* Exposed the asset filter callback.
This allows the slice converter to specifically load nested slices as opposed to not loading *any* referenced assets.
* Added support for multiply-nested slice instance conversion.

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/Instance/InstanceEntityIdMapper.cpp

@@ -154,7 +154,7 @@ namespace AzToolsFramework
             InstanceOptionalReference owningInstanceReference = m_storingInstance->m_instanceEntityMapper->FindOwningInstance(entityId);
 
             // Start with an empty alias to build out our reference path
-            // If we can't resolve this id we'll return a random new alias instead of a reference path
+            // If we can't resolve this id we'll return a new alias based on the entity ID instead of a reference path
             AliasPath relativeEntityAliasPath;
             if (!owningInstanceReference)
             {
@@ -162,7 +162,7 @@ namespace AzToolsFramework
                     "Prefab - EntityIdMapper: Entity with Id %s has no registered owning instance",
                     entityId.ToString().c_str());
 
-                return Instance::GenerateEntityAlias();
+                return AZStd::string::format("Entity_%s", entityId.ToString().c_str());
             }
 
             Instance* owningInstance = &(owningInstanceReference->get());