Flow configuration object to user code (.NET Core)

Right now, the recommended way for a CSX function to read app settings is through environment variables:
```
System.Environment.GetEnvironmentVariable(name, EnvironmentVariableTarget.Process);
```
This is not how one would handle this in ASP.NET, meaning it is difficult to share code between functions and web apps when referencing app settings.

We can get the ConfigurationManager for .NET full framework. We should also add the .NET Core equivalent for 2.0