Rust: Additional properties mixed & properties

[BelfordZ]

There are a few different types of object schemas

1. properties is used, additional props unused
2. props unused, addtional props used
3. both are used

at least for now, when additional props is used on its own, we should give the type HashMap<String, ..>.

If just props is used, it should be a simple struct.

If both are used, it should be a struct alongside a flattened AdditionalProps field.