--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

• [x] 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

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