`AsRef<Url>` as argument is practically unusable

Currently, [datafusion::execution::runtime_env::RuntimeEnv::object_store](https://docs.rs/datafusion/latest/datafusion/execution/runtime_env/struct.RuntimeEnv.html#method.object_store) takes an `impl AsRef`. Neither `Url` nor `&Url` implement this trait, so I am confused as to how I am supposed to use this. I ended up passing a `Box`, but I doubt that is the intended way to go. I think it should take a `&Url` instead. If `AsRef` is actually the intended type, then an example of how to use this would be nice.

Start with the RuntimeEnv::object_store API and its documentation at the linked docs.rs entry, then inspect how its AsRef argument is intended to be called. Done means either Url and &Url can be passed as expected or the documentation includes a clear working example of the intended usage.

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