Extract http headers for actix-protobuf

[jlloh]

PR Type

Feature

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the nightly rustfmt (cargo +nightly fmt).

Overview

Extract HeaderMap and pass it along so it can be used in the handler.

[jlloh]

Hello, I am trying to use the crate, but I realise that I need Header information from the http request as well for my purposes. I'm not sure whether it's more appropriate to put it in the ProtoBuf struct or the ProtoBufMessage struct, will adhere to your guidance.

Also note that I'm currently cloning the map which is probably not ideal, once I know which struct to put it in, I'll also work on avoiding the clone.

[jlloh]

Tagging @robjtede for comment.