Implement settings conversion traits (#4489)
## Summary
This does require cloning the settings, but I think it's fine. A better
solution would be to have owned and unowned settings structs, so that we
could convert `ResolverInstallerSettingsRef` to `InstallerSettingsRef`
without cloning, but that requires maintaining owned and unowned
variants.
Closes https://github.com/astral-sh/uv/issues/4455.
