Adding Caching Support

This client seems like a very thin wrapper over a HttpClient using dynamic. It would be nice to add many of the features in the Apollo JavaScript client such as client side caching.

You could use the [`IDistributedCache`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.caching.distributed.idistributedcache?view=aspnetcore-2.1) in the [Microsoft.Extensions.Caching.Abstractions](https://www.nuget.org/packages/Microsoft.Extensions.Caching.Abstractions/) NuGet package to store cached objects. That package has lots of providers e.g. In-Memory and Redis built for it already.