Add support for AppConfigDataClient in AWS SDK

[lavenderses]

What

Add new support for AppConfigDataClient.

closes #51.

How

AppConfig access costs money. So, I choose the way keeps the feature flags in local cache, not to request to it every flag evaluation.

CachedFeatureFlagManager

The core of local caching is CachedFeatureFlagManager.

This manages following things.

• local feature flags cache
• session for AWS AppConfig
• safe close of the resource (thread, connection to AWS AppConfig)

Also, an infastructure for executing infinite task is introduced.
This is ScheduledTaskExecutor.

Using this feature

Application Author who is using this provider implementatin can use this feature by configuring AwsAppConfigDataClientProxyConfig.
This proxy config is for using AwsAppConfigDataClientProxy, which proxys the feature flag request to AWS AppConfig through AppConfigDataClient.

e.g.

// provider configuration
AwsAppConfigClientOptions options = AwsAppConfigClientOptions.builder()
        .applicationName("AWS AppConfig application name")
        .environmentName("AWS AppConfig environment name")
        .profile("AWS AppConfig profile name")
        .awsAppConfigProxyConfig(
            // access AWS AppConfig via using `AppConfigDataClient`.
            AwsAppConfigDataClientProxyConfig.builder()
                // your aws region
                .region(Region.AP_NORTHEAST_1)
                // feature flags polling duration. this will affect AWS cost
                .pollingDelay(Duration.ZERO)
                .build()
        )
        .build();

// configure a provider
AwsAppConfigFeatureProvider provider = new AwsAppConfigFeatureProvider(options);

docs. https://sdk.amazonaws.com/java/api/2.25.45/software/amazon/awssdk/services/appconfigdata/AppConfigDataClient.html

Notes

Supporting use of AppConfigDataClient instance application author instantinated will be in the future.

[lavenderses]

LGTM