feat(v2): complete v2 reimplementation

tests/ssh-fixture/sshd_config

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+Port 2222
+ListenAddress 0.0.0.0
+HostKey /run/sshd/ssh_host_ed25519_key
+PidFile /run/sshd/sshd.pid
+AuthorizedKeysFile /run/sshd/authorized_keys
+UsePAM no
+PasswordAuthentication no
+KbdInteractiveAuthentication no
+ChallengeResponseAuthentication no
+PermitRootLogin no
+PermitEmptyPasswords no
+PubkeyAuthentication yes
+PermitUserEnvironment no
+AllowTcpForwarding no
+X11Forwarding no
+PermitTunnel no
+PermitTTY no
+GatewayPorts no
+AllowAgentForwarding no
+LogLevel VERBOSE
+Subsystem sftp internal-sftp
+
+Match User backup
+    ChrootDirectory /home/backup
+    ForceCommand internal-sftp
+    AllowTcpForwarding no
+    X11Forwarding no
+    PermitTTY no