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/docker/deployer/app/Dockerfile

@@ -6,4 +6,4 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
-
+RUN echo "Up!" > /usr/share/nginx/html/up

test/integration/docker/deployer/app_with_roles/Dockerfile

@@ -6,4 +6,4 @@ ARG COMMIT_SHA
 RUN echo $COMMIT_SHA > /usr/share/nginx/html/version
 RUN mkdir -p /usr/share/nginx/html/versions && echo "version" > /usr/share/nginx/html/versions/$COMMIT_SHA
 RUN mkdir -p /usr/share/nginx/html/versions && echo "hidden" > /usr/share/nginx/html/versions/.hidden
-
+RUN echo "Up!" > /usr/share/nginx/html/up

test/integration/docker/deployer/app_with_roles/config/deploy.yml

@@ -9,6 +9,11 @@ servers:
     hosts:
       - vm3
     cmd: sleep infinity
+proxy:
+  enabled: true
+  hosts:
+    - vm2
+  deploy_timeout: 2s
 
 asset_path: /usr/share/nginx/html/versions