This PR is a first step toward support for storing credentials in the system keyring. The keyring-rs crate is the best option for system keyring integration, but the latest version (v4) requires either that Linux users have libdbus installed or that it is built with libdbus vendored in. This is because v4 depends on dbus-secret-service, which was created as an alternative to secret-service so that users are not required to use an async runtime. Since uv does use an async runtime, this is not a good tradeoff for uv.

This PR:

• Vendors keyring-rs crate into a new uv-keyring workspace crate
• Moves to the async secret-service crate that does not require clients on Linux to have libdbus on their machines. This includes updating CredentialsAPI trait (and implementations) to use async methods.
• Adds uv-keyring tests to cargo test jobs. For cargo test | ubuntu, this meant setting up secret service and priming gnome-keyring as an earlier step.
• Removes iOS code paths
• Patches in @oconnor663 's changes from his keyring-rs PR
• Applies many clippy-driven updates