Updates in preparation for adding entity aliases to spawnables.

The following has been changed:
- AssetDataStream can now return the stored streaming deadline and priority.
- RootSpawnable now has an event that's called just before root spawnable spawns entities. This is an immediate event unlike the other events that are queued.

Signed-off-by: AMZN-koppersr <82230785+AMZN-koppersr@users.noreply.github.com>

Code/Framework/AzCore/AzCore/Asset/AssetDataStream.h

@@ -70,6 +70,9 @@ namespace AZ::Data
 
         const char* GetFilename() const override { return m_filePath.c_str(); }
 
+        AZStd::chrono::milliseconds GetStreamingDeadline() const { return m_curDeadline; }
+        AZ::IO::IStreamerTypes::Priority GetStreamingPriority() const { return m_curPriority; }
+
         // AssetDataStream specific APIs
 
         //! Whether or not all data has been loaded.