Improve typing of string enums

[pquentin]

Today, string enums are typed using Literal to allow specific values and None if the value is not set, which is OK. However, we also use str as an option, which allows any value and defeats the purpose of having types in the first place.

For example, see wait_for in https://elasticsearch-py.readthedocs.io/en/v8.13.1/api/ml.html#elasticsearch.client.MlClient.start_trained_model_deployment. The type is Literal['fully_allocated', 'started', 'starting'] | str | None which prevents any useful type checking. We should remove the str type.

Note that this is different from #1691 which is about using Python classes for enums.