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/integration_test.rb

@@ -169,10 +169,8 @@ class IntegrationTest < ActiveSupport::TestCase
       case app
       when "app"
         "127.0.0.1"
-      when "app_with_roles"
-        "localhost"
       else
-        raise "Unknown app: #{app}"
+        "localhost"
       end
     end
 end