Improve Pipeline `RequestContext.attribute` API to be typesafe

[anilgursel]

Comments from @akara:

I just looked up the RequestContext.attribute API in open source squbs and found it a bit loose. The rc.attribute call may pass a false type and cause a false cast in the RequestContext implementation. We can definitely do better by storing the type information. This is pretty much a case for extensible records, such as: https://github.com/milessabin/shapeless/wiki/Feature-overview:-shapeless-2.0.0#extensible-records