Implement structure type feature flag client

[lavenderses]

What

Implement conversion to Value, supporting Structure type feature flag value.

close #3

How

Expecting JSON format string value in flag_value.
Because of AWS AppConfig feature flag schema limitation (flag value cannot be nested), the spec for structure-type feature flag in this provider implementation is putting JSON string to flag_value attribute.

JSON Structure could be nested object. The value might be primitive like A, or nested JSON like B.

// A
{
  "{key name}": {
    "enable": true,
    // NOTE that this `flag_value` is primitive like number / string / boolean
    "flag_value": 12345
  }
}
// B
{
  "{key name}": {
    "enable": true,
    // NOTE that this `flag_value` is nested JSON
    "flag_value": {
      "foo": {
        "bar": 12345,
      },
      "qux": "quux"
    }
  }
}

So, the logic is converting feature flag string value to JsonNode, and then walk through DFS-ly and build nested Value. This is AppConfigParser#convertJsonNodeAsValueRecursively implementation.
See example in tests in AppConfigParserTest.ConvertJsonNodeAsValueRecursively class.

Note

This might confusing because some case might exist that geting feature flag value as structure unexpectedly, not as string.
But, this might never happen.
For example, some Application Authors want to get feature flag value as string (A in above example), not as structure.
In this case, they will call Evaluation API with getStringValue. So, type mismatching will not occur.

Notes

N/A

[lavenderses]

LGTM