Add push_env config

This setting allows you to automatically push env files when deploying.
The default is not to push any files, but you can set it to `all`,
`clear` or `secret` to push the relevant files.

The most useful setting is `clear` which will push the clear env files
every time you deploy.

In addition you can choose the env_type to push when calling
`kamal env push` directly:

```
kamal env push --env-type clear
kamal env push --env-type secret
kamal env push --env-type all # same as kamal env push
```

test/configuration_test.rb

@@ -317,4 +317,13 @@ class ConfigurationTest < ActiveSupport::TestCase
 
     assert_raises(ArgumentError) { Kamal::Configuration.new(@deploy_with_roles.merge(retain_containers: 0)) }
   end
+
+  test "push_env" do
+    assert_nil @config.push_env
+    assert_equal "all", Kamal::Configuration.new(@deploy.merge(push_env: "all")).push_env
+    assert_equal "clear", Kamal::Configuration.new(@deploy.merge(push_env: "clear")).push_env
+    assert_equal "secret", Kamal::Configuration.new(@deploy.merge(push_env: "secret")).push_env
+
+    assert_raises(ArgumentError) { Kamal::Configuration.new(@deploy_with_roles.merge(push_env: "foo")) }
+  end
 end