feat: C++ bindings for protobuf

[aakoshh]

Description

Problem*

Part of #7511
Builds on #7513

Summary*

Generates C++ bindings for the Protobuf schemas, to be used by Barretenberg.

I originally thought this would be convenient since we already have the acir.cpp and witness.cpp files generated, which contain the models necessary to deserialise the bincode encoded Program and WitnessMap, and at this point we already have protoc which we can invoke right there in the same step, always keeping the two in sync.

On second thought, this is probably best left to be done on the aztec-packages side: as long as the .proto schemas are available to that build, we can decide there which ones need to be built, and also make sure they compile (ie. that we have not forgotten anything, because unlike prost, protoc only generates source for what we tell it to do explicitly). It is also more conceptually sound because then the consumer decides what bindings to generate, rather we having to know exactly what bb can ignore (that is, we don't generate C++ program.proto and brillig.proto).

I'm opening the PR for the record, but I think we should close it and wait until we have synced the schemas over to aztec-packages and do it there.

We still have to generate acir.cpp and witness.cpp on the Noir side because it works with reflection, and in aztec-packages our job is to update e.g. acir.hpp, generate fresh Protobuf mappings, then manually do the mapping between them whenever something changes that we want to use on that side.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[aakoshh]

@TomAFrench do you agree that we close this and do it in aztec-packages instead?

[TomAFrench]

Hey, yeah I agree that once we have the proto file then we can leave it up to aztec-packages to generate their bindings.