Azure App Configuration for C++

**Is your feature request related to a problem? Please describe.**
Azure App Configuration works great in ASP.NET Core applications. It would be good if we could use it in C++ applications too.

**Describe the solution you'd like**
Add support for using Azure App Configuration to this SDK as a separate component.
- integration with Azure Key Vault would be great.
- support for primary and secondary App Configuration instance for high-availability.
- support for offline caching, i.e. it can store the "last known good" configuration encrypted on persistent storage.

**Describe alternatives you've considered**
Considering to use [Boost Asio](https://www.boost.org/doc/libs/release/libs/asio/) or [Boost Beast](https://www.boost.org/doc/libs/release/libs/beast/) to integrate with the Azure App Configuration REST API. Another alternative is to use [cpprestsdk](https://github.com/microsoft/cpprestsdk).

**Additional context**
- Azure App Configuration REST API: https://docs.microsoft.com/en-us/rest/api/appconfiguration/

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