Add pre and post app boot hooks

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.

test/test_helper.rb

@@ -36,6 +36,10 @@ class ActiveSupport::TestCase
   extend Rails::LineFiltering
 
   private
+    setup do
+      SSHKit::Backend::Netssh.pool.close_connections
+    end
+
     def stdouted
       capture(:stdout) { yield }.strip
     end