Set proxy run configuration in deploy YAML

[djmb]

With Kamal 2.0 the plan was to have kamal proxy as a zero config proxy, so that you could run multiple apps through a single proxy instance without configuration conflicts - as such a lot of configuration is done at deploy time, rather than boot time.

But there's actually quite a lot of configuration that's needed at boot time - anything that needs to be set where the container is started - ports, logging options, image version, custom docker options etc.

This can all be currently set with kamal proxy boot_config set ... but that is cumbersome and the config is stored on the servers which is not ideal. In practice you end up needing to set the config in a hook for consistency so it's in the app repo anyway.

So let's allow the config to be set in the deploy YAML at proxy/run instead. We'll keep the kamal proxy boot_config commands and use them if no run config is set, but we'll print a deprecation warning.

If you have conflicting proxy configs for a single host in an app, we'll raise a configuration error. If you have them across multiple apps, we'll just use them both and the container will use whichever config it was last booted with.

In the next major version we'll remove the boot_config commands and just use the run config.