Motivation and Context

This PR takes care of 4 longstanding issues with the Attributes/Managers subsystem :

• It separates the AbstractObject, Object and Stage attributes into separate source files. These were originally combined when the attributes/managers subsystem was in a more flat layout. Breaking them apart so they are easier to manage and understand and match their respective managers' structure.
• It removes an unnecessary abstract function path that was unused by most managers. Only Stage Attributes and Object Attributes should ever need to query whether an asset handle is a valid Primitive asset handle, so this removes the query from all managers other than those.
• It adds a function to ManagedFileBasedContainer to build a Managed object/Attributes directly from an appropriately configured string without needed to load a file from disk.
• Renames a file to match the class template it describes. -sigh-

How Has This Been Tested

All C /Python tests pass

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.