chore: Add GAPIC generator version selection to generator scripts
chore: Add GAPIC generator version selection to generator scripts

This will allow us to keep OwlBot and autosynth in sync in terms of
generator versions.

jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/gcloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet