feat(build): Add prostoc_args

[grippy]

Motivation

Compiling protobuf v3 files with optional fields requires setting Prost Config.protoc_args.

To enable compiling optional fields, some versions of protoc require the --experimental_allow_proto3_optional flag. This experimental flag was added in Protobuf 3.12. The recent Protobuf 3.15 release implies this flag is no longer required.

However, I have protoc v3.15.5 locally and I still get this error: protoc failed: msg.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set..

Solution

This change modifies the Tonic build config to store protoc_args and pass them to the Prost config prior to compiling protos.

[LucioFranco]

Thanks!