Add support for HAL media type

[ccheetham]

Using HAL removes the need to hardcode API endpoints in clients.

HAL resources for dotnet

https://docs.microsoft.com/en-us/archive/msdn-magazine/2013/january/asp-net-building-hypermedia-web-apis-with-asp-net-web-api
https://github.com/visualeyes/halcyon
https://github.com/JakeGinnivan/WebApi.Hal