Support appsync subscription

[lowecg]

As mentioned in issue #98, here are the changes necessary to support subscriptions with AppSync.

[oliyh]

Hi,

Thanks for this! I really appreciate the contribution. Do you know if the "subscriptions as data" is a common pattern, or is it something unique to Appsync? My inclination is to have something like an implementation "flavour" called Appsync, which would take care of any aspects unique to Appsync. The impl parts were intended more as a way of exposing the underlying transport libraries to empower users if they needed to tweak something at the transport level, but I see Appsync as more of a protocol thing.

What do you think?

Cheers

[lowecg]

Thank you for reviewing this, Oli.

I was trying to be agnostic in the naming of "subscription as data", but I'm not sure if it is a common pattern so the AppSync flavour makes sense. Noted on the usage of impl. I'll have a think about this and will put forward some changes this weekend.