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.
