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/main_test.rb

@@ -88,6 +88,14 @@ class MainTest < IntegrationTest
   end
 
   test "setup and remove" do
+    @app = "app_with_roles"
+
+    kamal :proxy, :set_config,
+      "--publish=false",
+      "--options=label=traefik.http.services.kamal_proxy.loadbalancer.server.scheme=http",
+      "label=traefik.http.routers.kamal_proxy.rule=PathPrefix\\\(\\\`/\\\`\\\)",
+      "label=traefik.http.routers.kamal_proxy.priority=2"
+
     # Check remove completes when nothing has been setup yet
     kamal :remove, "-y"
     assert_no_images_or_containers
@@ -123,6 +131,15 @@ class MainTest < IntegrationTest
     assert_proxy_not_running
   end
 
+  test "deploy with traefik" do
+    @app = "app_with_traefik"
+
+    first_version = latest_app_version
+
+    kamal :setup
+    assert_app_is_up version: first_version
+  end
+
   private
     def assert_envs(version:)
       assert_env :CLEAR_TOKEN, "4321", version: version, vm: :vm1