Add Swift GRPC Plugin To ZcashLightClientKit

[pacu]

We want the generated code from the proto files to be always in sync with current Swift-GRPC version

let's add the Swift-GRPC plugin to the SDK so that we can

• build generated code on the fly
• but still have reproducible builds

reference: https://github.com/grpc/grpc-swift

[Chlup]

This can't be done in a way we expected. Any generating of proto files depends on protoc binary. And this binary isn't distributed together with Swift-GRPC plugin. So if we do this we will bother all the developers that use SDK to have protoc installed even though they don't need it. Let's don't do this. At least not now.