GHSA-VP5Q-FF62-JWQV russh: Agent server never calls `Agent::confirm`, so keys added with the confirm constraint (`ssh-add -c`) sign without confirmation

Summary The agent server in russh::keys::agent::server never calls Agent::confirm before signing with a key that was added with the confirm constraint. That constraint is SSHAGENTCONSTRAINCONFIRM RFC 9987 §5.2.7.2, which is what ssh-add -c sends. The call was replaced by a hard-coded true in...