Introduce TreeEntry.TargetType and TreeEntryDefinition.TargetType
Obsolete TreeEntry.Type and TreeEntryDefinition.Type
Introduce ObjectType
Deploy ObjectType to OdbBackend

Sadly, this is a breaking change as there's no way to allow a migration path through the use of an [Obsolete] attribute.
Obsolete GitObjectType
Obsolete Tree.Trees and Tree.Blobs properties

Relying on this properties would trigger multiple enumerations of the
Tree entries. Exposing them is quite misleading for the consumer.