--Move AssetType enum class attributes namespace

[jturner65]

Motivation and Context

This PR moves the AssetType enum from the asset namespace to the metadata::attributes namespace where it can reside with the other metadata-related enum classes in AttributesEnumMaps. This is primarily for consistency but also to facilitate interactions with the enum-backed data in the Attributes that consume it.

The PR also moves the EMPTY_SCENE definition to Esp.h from Asset.h, in an effort to also maintain consistency in where our system-wide consts are stored.

How Has This Been Tested

Locally c++ and 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.

[aclegg3]

nice 👍