Provide a NuGet with functionality for reading usage against consumption app limits at runtime

The consumption plan for functions has a number of limits - one example is the number of concurrent outbound connections. It is actually possible to retrieve this information at runtime using a special environment variable that is updated dynamically however the code to do this is non-trivial. It would be most convenient for users if the code to parse this was wrapped up into a tiny standalone NuGet package. The relevant code is [here](https://github.com/Azure/azure-webjobs-sdk-script/blob/dev/src/WebJobs.Script/Scale/HostPerformanceManager.cs).