Enforce type of `_typ` to a single value instead of Enum type + default value

[lord-haffi]

Currently, it is theoretically possible (and therefore defined in the JSON-schemas) that a BO like Vertrag can hold a different value in _typ than "VERTRAG". This should be avoided. Instead, we should define the type like typ: Annotated[Literal["VERTRAG", None], Field(alias="_typ")] = "VERTRAG". We already set the default-value in every class, i.e. imo we don't profit from the enum pattern here.

This would also make the enum Typ obsolete. Optionally, we could create the enum dynamically by querying all Literal types in bo and com, but I don't think that we would need that.

Please note that it is currently impossible in Python to define Literals with enum members. It has to be a constant string.

What are your thoughts on this?
@hf-kklein, @hf-krechan, @hf-fvesely, @hf-aschloegl, @snsttr