Implement SAND-WP-0005: extension SDK and ext.vm-packer

Add SandboxExtension base class, extension SDK docs, vm-packer attach mode
for build-machines VMs, profile.vm-haskell-build, SSH port support, tests,
and migration docs.

tests/test_ssh.py

@@ -10,4 +10,10 @@ def test_destination_uses_ssh_config_when_user_unset() -> None:
 
 def test_destination_includes_explicit_user() -> None:
     ssh = SSHConfig(host="92.205.130.254", user="tegwick")
-    assert ssh.destination == "tegwick@92.205.130.254"
+    assert ssh.destination == "tegwick@92.205.130.254"
+
+
+def test_ssh_base_includes_port() -> None:
+    ssh = SSHConfig(host="localhost", user="build", port=12222)
+    assert "-p" in ssh.ssh_base()
+    assert "12222" in ssh.ssh_base()