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.

profiles/profile.vm-haskell-build.yaml

@@ -0,0 +1,35 @@
+id: profile.vm-haskell-build
+version: "1.0.0"
+extension: ext.vm-packer
+isolation:
+  level: microvm
+network:
+  default: deny
+  egress: []
+workspace:
+  mode: remote-canonical
+  access: rw
+scope_default: session
+ttl:
+  default: 8h
+  max: 24h
+  idle_reap: null
+resources:
+  cpu: null
+  memory_mb: null
+setup:
+  instructions: >
+    Requires a running build VM from the-custodian/infra/build-machines with SSH
+    tunnel or alias (e.g. haskell-build). Set SANDBOXER_VM_TUNNEL_PORT=12222 or
+    pass --input tunnel_port=12222.
+  secret_refs: []
+placement:
+  prefer: [localhost]
+  fallback: [workstation]
+reachability:
+  tunnel: ops-bridge
+  identity: ops-warden
+metadata:
+  cost_class: self-hosted
+  latency_class: standard
+  observability: none