Make CreateOpts fields public

Right now you either enumerate all the fields on a struct, or use default to get only defaults. Neither of those is super useful, most likely you just want a few fields set at a time and everything else default. Making fields public would enable the normal pattern of

```
GroupCreateOpts {
name: Some("word up".to_string()),
..Default::default()
}
```

Locate the Rust definition of GroupCreateOpts and the other CreateOpts structs referenced by this request. Read their current field visibility and Default implementations, then verify that callers can set selected fields with struct update syntax while retaining defaults. Run the relevant Rust test suite, if available, to confirm the public API compiles.

Written by the indexing model from the issue text.