feat: Change SSHCertificateSigner to be an async trait

[rustyconover]

Allow the sign() method of the SSHCertificateSigner trait to be asynchronous. Propagate the changes through the package to use Tokio.

I'm not sure you're going to want to merge this, but there is an advantage to allowing asynchronous things to happen when signing certificates.

[obelisk]

So I'm not thrilled using the feature flag in this way because of the breaking changes and that this is not currently stable but I do like the idea of providing an async signing API. Rustica actually had to get around this exact issue so that is why the add_signature method exists: https://github.com/obelisk/rustica/blob/x509_certificates/rustica/src/signing/amazon_kms.rs#L274

In Rustica I use async_trait when needed so I'd be more inclined to use that here. Alternatively perhaps having two traits, one which is async and can be pulled in separately to keep dependencies down could be a solution.