Configuration validation

Validate the Kamal configuration giving useful warning on errors.
Each section of the configuration has its own config class and a YAML
file containing documented example configuration.

You can run `kamal docs` to see the example configuration, and
`kamal docs <section>` to see the example configuration for a specific
section.

The validation matches the configuration to the example configuration
checking that there are no unknown keys and that the values are of
matching types.

Where there is more complex validation - e.g for envs and servers, we
have custom validators that implement those rules.

Additonally the configuration examples are used to generate the
configuration documentation in the kamal-site repo.

You generate them by running:

```
bundle exec bin/docs <kamal-site-checkout>
```

test/configuration/env_test.rb

@@ -19,7 +19,7 @@ class ConfigurationEnvTest < ActiveSupport::TestCase
 
   test "secret" do
     ENV["PASSWORD"] = "hello"
-    env = Kamal::Configuration::Env.from_config config: { "secret" => [ "PASSWORD" ] }
+    env = Kamal::Configuration::Env.new config: { "secret" => [ "PASSWORD" ] }
 
     assert_config \
       config: { "secret" => [ "PASSWORD" ] },
@@ -34,7 +34,7 @@ class ConfigurationEnvTest < ActiveSupport::TestCase
       "secret" => [ "PASSWORD" ]
     }
 
-    assert_raises(KeyError) { Kamal::Configuration::Env.from_config(config: { "secret" => [ "PASSWORD" ] }).secrets }
+    assert_raises(KeyError) { Kamal::Configuration::Env.new(config: { "secret" => [ "PASSWORD" ] }).secrets }
   end
 
   test "secret and clear" do
@@ -67,7 +67,7 @@ class ConfigurationEnvTest < ActiveSupport::TestCase
 
   private
     def assert_config(config:, clear:, secrets:)
-      env = Kamal::Configuration::Env.from_config config: config
+      env = Kamal::Configuration::Env.new config: config, secrets_file: "secrets.env"
       assert_equal clear, env.clear
       assert_equal secrets, env.secrets
     end