Wasteful clones of URLs made in various places

To work with Rust's borrow checker, we often have to make clones of URLs or other strings in order to capture in an `Fn` or `FnMut` e.g., `async move { }`. See https://github.com/Azure/typespec-rust/pull/867#pullrequestreview-3843450315 for one such place and discussion.

We could wrap it in an `Arc` but that also might be overkill. What other options might we have to avoid cloning potentially large strings?