[Key Vault] Add serialization method for models

Feature request based on issues such as https://github.com/Azure/azure-sdk-for-python/issues/17102 and https://github.com/Azure/azure-sdk-for-python/issues/41261.

Now that KV is generating with TypeSpec, we should be able to more easily support model serialization. Serialization should take some factors into account:

- Client-facing models generally have different attribute names than their underlying, generated counterparts
- Only the former attributes should be exposed
- Serialized models should be compatible as input to client methods, since DPG methods accept model parameters as JSON
- To support this, client methods will need to be able to convert serialized client-facing models into their generated counterparts (like we already do today with `to_generated` helpers and direct instantiation)

The latter point can probably be considered as a separate feature request; model serialization can be a separate feature from round-tripping. When active work on this issue begins, a separate issue can be appropriately created if necessary.