Add kamal-proxy in experimental mode

The proxy can be enabled via the config:

```
proxy:
  enabled: true
  hosts:
    - 10.0.0.1
    - 10.0.0.2
```

This will enable the proxy and cause it to be run on the hosts listed
under `hosts`, after running `kamal proxy reboot`.

Enabling the proxy disables `kamal traefik` commands and replaces them
with `kamal proxy` ones. However only the marked hosts will run the
kamal-proxy container, the rest will run Traefik as before.

test/integration/proxy_test.rb

@@ -0,0 +1,84 @@
+require_relative "integration_test"
+
+class ProxyTest < IntegrationTest
+  setup do
+    @app = "app_with_roles"
+  end
+
+  test "boot, reboot, stop, start, restart, logs, remove" do
+    kamal :envify
+
+    kamal :proxy, :boot
+    assert_proxy_running
+
+    output = kamal :proxy, :reboot, "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-traefik-reboot", "post-traefik-reboot"
+    assert_match /Rebooting Traefik on vm1,vm2.../, output
+    assert_match /Rebooted Traefik on vm1,vm2/, output
+
+    output = kamal :proxy, :reboot, "--rolling", "-y", "--verbose", capture: true
+    assert_proxy_running
+    assert_hooks_ran "pre-traefik-reboot", "post-traefik-reboot"
+    assert_match /Rebooting Traefik on vm1.../, output
+    assert_match /Rebooted Traefik on vm1/, output
+    assert_match /Rebooting Traefik on vm2.../, output
+    assert_match /Rebooted Traefik on vm2/, output
+
+    kamal :proxy, :boot
+    assert_proxy_running
+    assert_traefik_running
+
+    # Check booting when booted doesn't raise an error
+    kamal :proxy, :stop
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    # Check booting when stopped works
+    kamal :proxy, :boot
+    assert_proxy_running
+    assert_traefik_running
+
+    kamal :proxy, :stop
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    kamal :proxy, :start
+    assert_proxy_running
+    assert_traefik_running
+
+    kamal :proxy, :restart
+    assert_proxy_running
+    assert_traefik_running
+
+    logs = kamal :proxy, :logs, capture: true
+    assert_match /Traefik version [\d.]+ built on/, logs
+
+    kamal :proxy, :remove
+    assert_proxy_not_running
+    assert_traefik_not_running
+
+    kamal :env, :delete
+  end
+
+  private
+    def assert_proxy_running
+      assert_match /basecamp\/kamal-proxy:latest   \"kamal-proxy run\"/, proxy_details
+    end
+
+    def assert_proxy_not_running
+      assert_no_match /basecamp\/kamal-proxy:latest   \"kamal-proxy run\"/, proxy_details
+    end
+
+    def assert_traefik_running
+      assert_match /traefik:v2.10   "\/entrypoint.sh/, proxy_details
+    end
+
+    def assert_traefik_not_running
+      assert_no_match /traefik:v2.10   "\/entrypoint.sh/, proxy_details
+    end
+
+    def proxy_details
+      kamal :proxy, :details, capture: true
+    end
+end