Allow ssh/config to additionally accept a string, or an array of strings

This is accepted by Net::SSH, research done by @jeremy in #908 (comment)

This is already documented as working correctly in https://github.com/basecamp/kamal/blob/74a06b0ccda616c86ebe1729d0795f39bcac9f00/lib/kamal/configuration/docs/ssh.yml#L65-L70

However, before this change only booleans were allowed because of the example configuration file.

Author: Burgestrand

lib/kamal/configuration/ssh.rb

@@ -7,7 +7,7 @@ class Kamal::Configuration::Ssh
 
   def initialize(config:)
     @ssh_config = config.raw_config.ssh || {}
-    validate! ssh_config
+    validate! ssh_config, with: Kamal::Configuration::Validator::Ssh
   end
 
   def user

lib/kamal/configuration/validator/ssh.rb

@@ -0,0 +1,25 @@
+class Kamal::Configuration::Validator::Ssh < Kamal::Configuration::Validator
+  BOOLEAN_OR_STRING_OR_ARRAY_OF_STRING_KEYS = [ "config" ]
+  SPECIAL_KEYS = BOOLEAN_OR_STRING_OR_ARRAY_OF_STRING_KEYS
+
+  def validate!
+    validate_against_example! \
+      config.except(*SPECIAL_KEYS),
+      example.except(*SPECIAL_KEYS)
+
+    BOOLEAN_OR_STRING_OR_ARRAY_OF_STRING_KEYS.each do |key|
+      value = config[key]
+
+      with_context(key) do
+        validate_type! value, TrueClass, String, Array
+        validate_array_of!(value, String) if value.is_a?(Array)
+      end
+    end
+  end
+
+  private
+
+  def special_keys
+    @special_keys ||= config.keys & SPECIAL_KEYS
+  end
+end