Share flag_value JsonNode extraction logic through each extended class

[lavenderses]

What

Use shared flag_value getting method through all extenion parser for each feature flag type.

Why

Thanks to #7, parser logic is separeted to each feature flag type.
But, this provider's expected JSON schema from AWS AppConfig is same (following schema, see also AppConfigValue).

So, the logic which get flag_value value as JsonNode should be same, but now it is separeted.

{
  // feature flag key name in OpenFeature world
  "{key name}": {
    // is this feature flag enabled in AWS AppConfig world
    "enabled": boolean,
    // the actual feature flag value in OpenFeature world and this provider implementation world
    "flag_value": number | string | boolean | regex
  }
}

How

Move extraction logic to AbstractAttributeParser#getValidFlagValueNode. AbstractAttributeParser is the abstract implementation for each feature flag type.
Extended parser class use this to get flag_value value as JsonNode.

Only Object type feature flag does not have expected JsonNode type, so this null check is introduced.
https://github.com/lavenderses/AWSAppConfig-openfeature-provider-java/blob/54c934a29737cfc00fb5be9fa24fcf3ab63715a0/src/main/java/io/github/lavenderses/aws_app_config_openfeature_provider/parser/AbstractAttributeParser.java#L98

Notes

N/A

[lavenderses]

LGTM