zitadel abstraction is leaky

[tlater-famedly]

The zitadel crate has a lot of internal data structs which we just pub use since they're necessary for using various functions: https://github.com/famedly/zitadel-rust-client/blob/main/src/lib.rs#L53

This is not per-se a problem, but we should either fully commit and expose them in our Config (with adequate serde derivations) or cleanly hide them behind abstractions. The latter is obviously better practice, but the purpose of this crate is effectively just to tape over the sins of code generation, rather than to provide any real further abstraction, so that would be a lot of empty plumbing code.

We need to decide which way to go and follow that more consistently.