Unify manifest reading/writing on an a single API

[mmitche]

There are quite a few places that read and write asset manifests between Arcade, UB infrastructure, and Maestro/Darc functionality. We would ideally unify these on one. There is one already that is a decent candidate in arcade (VersionTools).

Existing known reading/writing of manifests

• MSBuild tasks in UB's eng/tools/Microsoft.DotNet.UnifiedBuild.Tasks
• Arcade promotion pipeline
• Arcade build manifest creation
• Gather-drop?
• Release pipeline?