authenticate_openssh_cert only support RSA-SHA-512 for private key signing

[lanshubo]

authenticate_openssh_cert currently hardcodes RSA-SHA-512 for RSA private key signing, causing failures against legacy servers that only accept RSA-SHA-1.

// in ssh-key, this will forcly use RSA-SHA-512 to sign
signature::Signer::try_sign(key.deref(), buffer)?
    .encoded()?
    .encode(&mut *buffer)?;

Raised a new PR #678 to add a new method variant that accepts a PublicKeyOrCertificate argument so callers can control the signing algorithm explicitly.