draft: implemented alternative protobuf implementation

[t2m4k1]

PR Type

Refactor

PR Checklist

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

Overview

Generally I'd like to discuss whether a ProtoBuf implementation for actix should be closer to the existing JSON implementation. Original JSON implementation

For that reason I created this alternative ProtoBuf implementation.

• Introduced compression
• Extended ProtoConfig by allowing the definition of custom content-types & error-handler
• Ported over most testcases from JSON implementation
• Aligned code with preexisting JSON implementation
• Not using LocalBoxFuture anymore

Since I'm rather new to programming in Rust I'd definitely love to hear your inputs on this. I guess there might be some pitfalls I'm unaware of! 😄

Thanks for having a look