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/fixtures/deploy_with_low_percentage_boot_strategy.yml

@@ -1,19 +0,0 @@
-service: app
-image: dhh/app
-servers:
-  web:
-    - "1.1.1.1"
-    - "1.1.1.2"
-  workers:
-    - "1.1.1.3"
-    - "1.1.1.4"
-builder:
-  arch: amd64
-
-registry:
-  username: user
-  password: pw
-
-boot:
-  limit: 1%
-  wait: 2

test/fixtures/deploy_with_percentage_boot_strategy.yml

@@ -1,19 +0,0 @@
-service: app
-image: dhh/app
-servers:
-  web:
-    - "1.1.1.1"
-    - "1.1.1.2"
-  workers:
-    - "1.1.1.3"
-    - "1.1.1.4"
-builder:
-  arch: amd64
-
-registry:
-  username: user
-  password: pw
-
-boot:
-  limit: 1%
-  wait: 2