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/cli/cli_test_case.rb

@@ -40,8 +40,9 @@ class CliTestCase < ActiveSupport::TestCase
         .with(:docker, :buildx, :inspect, "kamal-local-docker-container")
     end
 
-    def assert_hook_ran(hook, output, version:, service_version:, hosts:, command:, subcommand: nil, runtime: false, secrets: false)
-      assert_match %r{usr/bin/env\s\.kamal/hooks/#{hook}}, output
+    def assert_hook_ran(hook, output, count: 1)
+      regexp = ([ "/usr/bin/env .kamal/hooks/#{hook}" ] * count).join(".*")
+      assert_match /#{regexp}/m, output
     end
 
     def with_argv(*argv)