feat(gnarkd): exposing gnark APIs through RPCs

[gbotrel]

This is a placeholder.

cmd/gnarkd will expose through some gRPC APIs groth16.Prove and groth16.Verify. It will handle large circuits and large witness, and be easily callable from non-Go codebases.

--> gnarkd/README.md contents:
WARNING: this is an experimental feature and might not stay in gnark main repository in the future.

gnarkd

gnarkd is a gnark Proving / Verifying server.
It offers synchronous gRPC APIs (use when proving time & witness size are short) and asynchronous APIs.

See pb/gnardk.proto for up to date protobuf service description.

gRPC clients can be generated for multiple languages (Go, Rust, ...) see protoc doc for more info.
In go:

protoc --experimental_allow_proto3_optional --go_out=. --go_opt=paths=source_relative --go-grpc_out=. --go-grpc_opt=paths=source_relative  pb/gnarkd.proto

Under the hood

When gnarkd starts, it loads the circuits defined in circuits/ folder. Circuits must be stored in a separate folder, under a curve subfolder.
Example:

• circuits/bn256/cubic will contain cubic.pk, cubic.vk and cubic.r1cs.
• CircuitID (as needed in the APIs) is then bn256/cubic

gnarkd listens on 2 distinct TCP connections: one for gRPC, one for receiving large witnesses on async calls.

On this second connection, the server expects: jobID | witness .

• jobID is returned by CreateProveJob and is a standard UUID (RFC 4122) on 16 byte (server impl uses github.com/google/uuid)
• gnarkd knows which witness size to expect (via r1cs.GetNbPublicWires, r1cs.GetNbSecretWires and r1cs.SizeFrElement)

Example client (Go)

See client/example.go.