remove dependency on gapic-generator-go

Today gapic-generator-go contains code that generates the CLI for showcase. This code should be migrated to this repo to help break the interdependency issues between the two projects: https://github.com/googleapis/gapic-generator-go/tree/main/cmd/protoc-gen-go_cli

Start by reviewing cmd/protoc-gen-go_cli in gapic-generator-go and locating how gapic-showcase currently depends on it. Identify the corresponding integration points in this repository, then verify that the CLI generator is available here without the dependency on gapic-generator-go.

Written by the indexing model from the issue text.