Return full asset on Manifest get

[donaldgray]

Reason for change was to populate "assets" on manifest GET with full details from DLCS.

Refactored some classes to accomodate this:

• Renamed ManifestService -> ManifestWriteService and introduced an interface
• Moves GetAssets from ManifestService to a new class, ManifestReadService, to allow it to be shared
• Refactored code from GetManifest handler into ManifestReadService, and updated it to call GetAssets to read Asset data from DLCS.