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

This gets passed through to SSHKit options. See basecamp#908

Documentation mentions a string or array of strings are allowed, but they are not because:
* Validator example supplies a boolean, so only true/false are enabled.
* `Kamal::Configuration::Ssh#options` doesn't expose `config`, used in lib/kamal/commander.rb:167

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
@@ -38,8 +38,12 @@ def key_data
     ssh_config["key_data"]
   end
 
+  def config
+    ssh_config["config"]
+  end
+
   def options
-    { user: user, port: port, proxy: proxy, logger: logger, keepalive: true, keepalive_interval: 30, keys_only: keys_only, keys: keys, key_data: key_data }.compact
+    { user: user, port: port, proxy: proxy, logger: logger, keepalive: true, keepalive_interval: 30, keys_only: keys_only, keys: keys, key_data: key_data, config: config }.compact
   end
 
   def to_h

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