Support JSON / YAML schema object flag as Value

[lavenderses]

What

Thanks to #1 and #2, this provider supports boolean feature flas.

Next, this provider implementation should support structure type feature flag.

Because of AWS AppConfig feature flag schema limitation (flag value cannot be nested), how about putting JSON string to flag_value attribute?
I think this is much consistent to other feature flag type...
(In the future, suporting free form configuration is also good,,,
https://docs.aws.amazon.com/appconfig/latest/userguide/appconfig-free-form-configurations-creating.html)

{
  "{key name}": {
    "enabled": boolean,
    // "Attribute (optional)"
    "flag_value": { // JSON format
      "foo": "bar",
    }
  }
}

This is the requiremetns 2.2.2.1.

The feature provider interface MUST define methods for typed flag resolution, including boolean, numeric, string, and structure.

Notes

N/A