Configurable Kamal directory

To avoid polluting the default SSH directory with lots of Kamal config,
we'll default to putting them in a `kamal` sub directory.

But also make the directory configurable with the `run_directory` key,
so for example you can set it as `/var/run/kamal/`

The directory is created during bootstrap or before any command that
will need to access a file.

lib/kamal/commander.rb

@@ -116,6 +116,10 @@ class Kamal::Commander
     @registry ||= Kamal::Commands::Registry.new(config)
   end
 
+  def server
+    @server ||= Kamal::Commands::Server.new(config)
+  end
+
   def traefik
     @traefik ||= Kamal::Commands::Traefik.new(config)
   end