Python backing store support

[samwelkanda]

This issue is created to track remaining work to enable the backing store by default for Python clients.

The main idea is to store values behind the models in another memory space instead of using fields/properties.
This enables scenarios like dirty tracking, data sync with other data sources and much more. More information can be found in our conceptual documentation

Todo

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1. Add a backing store interface in the abstractions
2. Add a backed model interface in the abstractions
3. Add an in memory backing store implementation in the abstractions
4. Add a backing store factory interface in the abstractions
5. Implement the factory for the in memory backing store
6. Add a singleton for the backing store factory in the abstractions
7. Add event hooks for the json parse node (on before, on after, etc...)
8. Call the hooks when provided in the JSON implementation
9. Copy the hooks when provided in the JSON implementation
10. Implement factories for proxy
11. implement those factories for the backing store