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/test_helper.rb

@@ -13,6 +13,13 @@ ActiveSupport::LogSubscriber.logger = ActiveSupport::Logger.new(STDOUT) if ENV["
 # Applies to remote commands only.
 SSHKit.config.backend = SSHKit::Backend::Printer
 
+class SSHKit::Backend::Printer
+  def upload!(local, location, **kwargs)
+    local = local.string.inspect if local.respond_to?(:string)
+    puts "Uploading #{local} to #{location} on #{host}"
+  end
+end
+
 # Ensure local commands use the printer backend too.
 # See https://github.com/capistrano/sshkit/blob/master/lib/sshkit/dsl.rb#L9
 module SSHKit