Proxy boot config

Add commands for managing proxy boot config. Since the proxy can be
shared by multiple applications, the configuration doesn't belong in
`config/deploy.yml`.

Instead you can set the config with:

```
Usage:
  kamal proxy boot_config <set|get|clear>

Options:
      [--publish], [--no-publish], [--skip-publish]   # Publish the proxy ports on the host
                                                      # Default: true
      [--http-port=N]                                 # HTTP port to publish on the host
                                                      # Default: 80
      [--https-port=N]                                # HTTPS port to publish on the host
                                                      # Default: 443
      [--docker-options=option=value option2=value2]  # Docker options to pass to the proxy container
```

By default we boot the proxy with `--publish 80:80 --publish 443:443`.

You can stop it from publishing ports, specify different ports and pass
other docker options.

The config is stored in `.kamal/proxy/options` as arguments to be passed
verbatim to docker run.

Where someone wants to set the options in their application they can do
that by calling `kamal proxy boot_config set` in a pre-deploy hook.

There's an example in the integration tests showing how to use this to
front kamal-proxy with Traefik, using an accessory.

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

@@ -0,0 +1,29 @@
+service: app_with_traefik
+image: app_with_traefik
+servers:
+  - vm1
+  - vm2
+deploy_timeout: 2
+drain_timeout: 2
+readiness_delay: 0
+
+registry:
+  server: registry:4443
+  username: root
+  password: root
+builder:
+  driver: docker
+  arch: <%= Kamal::Utils.docker_arch %>
+  args:
+    COMMIT_SHA: <%= `git rev-parse HEAD` %>
+accessories:
+  traefik:
+    service: traefik
+    image: traefik:v2.10
+    port: 80
+    cmd: "--providers.docker"
+    options:
+      volume:
+        - "/var/run/docker.sock:/var/run/docker.sock"
+    roles:
+      - web