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/integration/app_test.rb

@@ -15,7 +15,9 @@ class AppTest < IntegrationTest
     # kamal app start does not wait
     wait_for_app_to_be_up
 
-    kamal :app, :boot
+    output = kamal :app, :boot, "--verbose", capture: true
+    assert_match "Booting app on vm1,vm2...", output
+    assert_match "Booted app on vm1,vm2...", output
 
     wait_for_app_to_be_up
 

test/integration/docker/deployer/app/.kamal/hooks/post-app-boot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Booted app on ${KAMAL_HOSTS}..."
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/post-app-boot

test/integration/docker/deployer/app/.kamal/hooks/pre-app-boot

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Booting app on ${KAMAL_HOSTS}..."
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/pre-app-boot

test/integration/main_test.rb

@@ -8,19 +8,19 @@ class MainTest < IntegrationTest
 
     kamal :deploy
     assert_app_is_up version: first_version
-    assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "post-deploy"
+    assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "pre-app-boot", "post-app-boot", "post-deploy"
     assert_envs version: first_version
 
     second_version = update_app_rev
 
     kamal :redeploy
     assert_app_is_up version: second_version
-    assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "post-deploy"
+    assert_hooks_ran "pre-connect", "pre-build", "pre-deploy", "pre-app-boot", "post-app-boot", "post-deploy"
 
     assert_accumulated_assets first_version, second_version
 
     kamal :rollback, first_version
-    assert_hooks_ran "pre-connect", "pre-deploy", "post-deploy"
+    assert_hooks_ran "pre-connect", "pre-deploy", "pre-app-boot", "post-app-boot", "post-deploy"
     assert_app_is_up version: first_version
 
     details = kamal :details, capture: true