Add two new hooks pre-app-boot and post-app-boot. They are analagous
to the pre/post proxy reboot hooks.
If the boot strategy deploys in groups, then the hooks are called once
per group of hosts and `KAMAL_HOSTS` contains a comma delimited list of
the hosts in that group.
If all hosts are deployed to at once, then they are called once with
`KAMAL_HOSTS` containing all the hosts.
It is possible to have pauses between groups of hosts in the boot config,
where this is the case the pause happens after the post-app-boot hook is
called.
cd73cea850