Re-evaluate our usage of `str`, `String`, etc. in `slicec-cs` API

[InsertCreativityHere]

We have a variety of function signatures ranging from simple &str to T: ToString to impl Into<String>. We should look through our API surface and see if these types could be improved (for greater flexibility on the calling side), or simplified in cases where it's use is limited.

See https://github.com/icerpc/icerpc-csharp/pull/3925#discussion_r1487969037