Enable real-time consistency capability by adding support for sync-tokens in AppConfiguration

If we follow precedence from other languages, this will require a SyncTokenPolicy, with a caching mechanism, which does the appropriate thing with the sync-token headers. The current thinking is to hand-modify the SDK to add support for this on top of the generated code (returned as part of operation responses).

This also needs a client-level public helper methods for customers to use, to complete the scenario:
- `ConfigurationClient::UpdateSyncToken(std::string token);`

Concept Doc for reference:
https://learn.microsoft.com/en-us/azure/azure-app-configuration/rest-api-consistency

Examples from other SDKs:
- [HTTP Policy GoLang](https://github.com/Azure/azure-sdk-for-go/blob/27afdb5ce01da51bc9d0b20f5e2b027a82de9553/sdk/data/azappconfig/internal/synctoken/policy.go#L31)
- [Type and Parser in GoLang](https://github.com/Azure/azure-sdk-for-go/blob/27afdb5ce01da51bc9d0b20f5e2b027a82de9553/sdk/data/azappconfig/internal/exported/sync_token.go#L33)
- [HTTP Policy .NET](https://github.com/Azure/azure-sdk-for-net/blob/c93bc3c485c3dae1baf4af8fb8ab5334fa09c8d1/sdk/appconfiguration/Azure.Data.AppConfiguration/src/SyncTokenPolicy.cs#L11)