Determine whether ConfigurationSetting is special type

It seems like .NET SDK exposes the following types: [SecretReferenceConfigurationSetting](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/appconfiguration/Azure.Data.AppConfiguration/src/Models/SecretReferenceConfigurationSetting.cs) and [FeatureFlagConfigurationSetting](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/appconfiguration/Azure.Data.AppConfiguration/src/Models/FeatureFlagConfigurationSetting.cs)

[Examples](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/appconfiguration/Azure.Data.AppConfiguration/samples/Sample10_SecretReference.md#retrieve-a-secret-reference)

Currently, the .NET provider hard-coded the feature flag prefix and content types of snapshot reference, secret reference and feature flag and use them to parse the configuration setting type manually.

The better way is to use the APIs from .NET SDK to parse the configuration setting.