[Go] New Module for Generated Code

[Victor Barua]

As part of https://github.com/substrait-io/substrait-go/issues/128 we're planning on moving the generated Go protobuf code out of the substrait-go repo and into a new substrait-protobuf repo.

At the moment, the (new) module is defined as

module github.com/substrait-io/substrait-protobuf/go

and imports of the protobufs in Go would look like

"github.com/substrait-io/substrait-protobuf/go/substraitpb"

We wanted to check in with any Go users to see if you had any suggestions or improvements for this structure.

You can see the proposed layout of the generated Go code in https://github.com/substrait-io/substrait-protobuf/tree/main/go, and what the usage of it looks like in https://github.com/substrait-io/substrait-go/pull/127.