`&CStr` variants of functions using `&str`

For FFI, we convert most &strs to &CStr (and the other way around) to get a c-style string that the pipewire library expects/provides, which isn't a zero cost conversion.

Add *_cstr variants to functions, callbacks, iterators, etc. so that C strings can be passed and obtained directly instead of having to convert back and forth.

For examples where this was already done, see !210 (merged), and spa::utils::dict::DictRef::iter_cstr().