Add a pre-init hook

The hook is run before the environment is loaded or the config is
parsed.

This makes it a bit of a special case - it doesn't have the usual
KAMAL_XYZ environment variables, as we haven't parsed the config.

The use case for this is to do auth checking or setup. So for example
we can confirm you are logged in to a secret manager, and then you
can directly call it to load your secrets in the .kamal/.env file
using .dotenv's
[command substitution](https://github.com/bkeepers/dotenv?tab=readme-ov-file#command-substitution).

test/test_helper.rb

@@ -26,6 +26,10 @@ end
 class ActiveSupport::TestCase
   include ActiveSupport::Testing::Stream
 
+  setup do
+    Kamal::Cli::Base.ran_pre_init_hook = false
+  end
+
   private
     def stdouted
       capture(:stdout) { yield }.strip