Integrate the existing signature API/FFI with the new gRPC/Protobuf microservice and Cgo

[l0k18]

Describe what you want implemented
Integrate the existing signature API/FFI with the new gRPC/Protobuf microservice and Cgo

Describe "Why" this is needed
Currently the bls12_381-sign-go repository has a makefile that clones this one into it. This won't be neccessary now, relative paths can be used instead. At the same time, this decouples the gRPC service from the bls12_381-sign FFI used by Cgo, and it means that once this repository is cloned, all of the BLS12_381 signature API access methods are all in one place.

Describe alternatives you've considered
I started to put the Rust gRPC service into bls12_381-sign-go but then ran into the problem of importing the Rust internal functions. The gRPC service was initially created inside the main crate in this repository but this greatly complicated the build for the rust based FFI.

Additional context
Add any other context or screenshots about the feature request here.