Add appsetting.json support

[milkshakeuk]

Currently App.config is used for configuring various things within the project.

• Hl7PackageCollection used for adding custom packages to the PackageManager so the PipeParser can find the custom definitions.
• UseFactory changes some of the generated source code
• ConnectionString used in the source code generator

This ticket is to add support for appsettings.json which is used for configuration in modern dotnet applications.