Generic Response mapping using Expr

[afsalthaj]

Fixes #316

• Flexible mapping representation using Expr
• Although the mapping corresponding to a protocol is parsed during evaluation, a validation is done when we submit the API definition using open API spec as well.
• There isn't a performance implication for trying to parse it during evaluation, because Expr is already parsed, and all that's done pattern matching on whether it is Expr::Record, and fetch the status, body and header out of it which is a constant big O.

[afsalthaj]

Note this is a method available on WorkerReponse as to_http_response called only for the Http Request endpoint

[afsalthaj]

Not waiting for the worker-executor tests, as the change don't impact this at all.