Add a mutex around loading secrets

Loading secrets may ask for use input, so we need to ensure only one
thread does it at a time.

lib/kamal/configuration.rb

@@ -9,7 +9,7 @@ class Kamal::Configuration
   delegate :service, :image, :labels, :stop_wait_time, :hooks_path, to: :raw_config, allow_nil: true
   delegate :argumentize, :optionize, to: Kamal::Utils
 
-  attr_reader :destination, :raw_config
+  attr_reader :destination, :raw_config, :secrets
   attr_reader :accessories, :aliases, :boot, :builder, :env, :healthcheck, :logging, :traefik, :servers, :ssh, :sshkit, :registry
 
   include Validation
@@ -64,6 +64,8 @@ class Kamal::Configuration
     @ssh = Ssh.new(config: self)
     @sshkit = Sshkit.new(config: self)
 
+    @secrets = Kamal::Secrets.new(destination: destination)
+
     ensure_destination_if_required
     ensure_required_keys_present
     ensure_valid_kamal_version