Add new uv-keyring crate that vendors the keyring-rs crate

[jtfmumm]

Work in progress

Includes:

• Vendoring keyring-rs crate into new uv-keyring workspace crate
• Moving to the async secret-service crate that does not require clients on Linux to have libdbus on their machines
• Removing iOS code paths
• Patching in @oconnor663 's changes from his keyring-rs PR

TODO:

• Add keyring tests to CI
• Determine if we still want the threading tests (currently commented out)
• Document unsafe code
• Improve PR description

[jtfmumm]

We need to determine if we still want these threading tests. They were created for the sync version of these functions in the original crate.