Allow --metrics_port and --debug options to be set via the boot config.
--metrics_port support will come in kamal-proxy v0.8.8, so this option
doesn't work right now.
This will be updated before the next Kamal release though and we can add
integration tests for the metrics at that point.
If there are accessories defined in the configuration, we'll not require
servers to be defined as well.
This allows for accessory-only configurations which allows you to run
external images with kamal-proxy for zero-downtime deployments.
We don't manage image cleanup for accessories though so the user will
need to deal with that themselves.
Use the --registry, --repository and --image_version options of
`kamal proxy boot_config set` to change the kamal-proxy image used.
We'll still insist that the image version is at least as high as the
minimum.
