Add SSH SFTP backend support

examples/ssh-destination.yml

@@ -0,0 +1,21 @@
+# Environment-gated example.
+# Replace host, user, path, ssh_key_file, and known_hosts with values for an
+# SSH/SFTP endpoint you control before running this config.
+pipelines:
+  - id: example-ssh-destination
+    source:
+      backend: local
+      path: examples/source-bundle
+    destinations:
+      - id: ssh-archive
+        backend: ssh
+        host: ssh.example.com
+        user: distributor
+        port: 22
+        path: /srv/distributor/archive
+        ssh_key_file: /home/distributor/.ssh/id_ed25519
+        known_hosts: /home/distributor/.ssh/known_hosts
+        host_key_policy: strict
+        publish:
+          source: true
+          html: false