Add support for keyring v4

[brotskydotcom]

Hi @complexspaces, remember me? I'm the maintainer of the keyring crate. With the impending release of v4 of keyring, its dependency on this crate is being discontinued in favor of a synchronous Secret Service client.

There are a few clients of keyring who are already using async and the zbus, and they might like to keep using this crate to access the Secret Service, so I would like to move that credential store implementation into this crate (which it depended on). That way clients can use the keyring extensibility mechanisms to use this crate to access the secret service instead of keyring's built-in keystore.

I'm going to submit a PR which adds the keyring credential store implementation as a module in this crate. It's completely backward-compatible, because the new functionality is hidden behind a new "keystore" feature. It would be great if you could review the PR and, once I've made any changes you like, integrate it into a release of this crate.

Thanks in advance for taking a look!