Implement identity mapping.

[aurelien-semence]

Implement identity from context as second parameter like describe here: https://docs.aws.amazon.com/appsync/latest/devguide/resolver-context-reference.html

[sbstjn]

Awesome. Looks nice, thanks for the contribution! 🎉

I'll try to review and merge it later today.

[connorpower]

@sbstjn There's not been any activity on this issue for a while — but the functionality would be awesome. Anything I could do to help?

[demiban]

@sbstjn Any plans on getting this PR merged? It would be a great enhancement.