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](https://github.com/open-source-cooperative/dbus-secret-service),
which was created as an alternative to
[secret-service](https://github.com/open-source-cooperative/secret-service-rs)
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](https://github.com/open-source-cooperative/keyring-rs/pull/261)
* Applies many clippy-driven updates
