[core] change serialization for hybrid models to `model_dump` in `azure.core.serialization`

We want to follow what dataclass does and do model -> json serialization as a module-level method in `azure.core.serialization`. The thought here is to add something like `model_dump` in `azure.core.serialization`, which will take in a hybrid model and output it in a dumped model format. This way, people can save it to file / pass to `json.dumps`.

As part of this, we will deprecate `.as_dict()` on our new hybrid models