Enhance polymorphic deserialization to handle multi-level inheritance

Currently the deserializer cannot instantiate types that are not direct descendants of the declared return type. Instead it will default to the base type.

This was a performance compromise because in Microsoft Graph all types derive from a base type "entity" and if any operation returned an entity it would be expensive to search the entire tree of types looking for the correct type.

One solution to consider in the future is to create a global mapping table that is aware that Graph has globally unique discriminators so that it can reduce the duplication of mapping tables. We would then move the type mappings for the types into request builders and property deserializers.

Start by locating the deserializer and its type-mapping logic, then review how request builders and property deserializers currently define mappings. Compare the behavior for direct and multi-level descendants, and define completion as correctly instantiating multi-level types without an impractical search across the entire type tree.

Written by the indexing model from the issue text.