Add synth.py for Bigtable
Add synth.py for Bigtable

This is probably the most challenging API to generate, as it uses Roslyn. Once this works, we'll generate synth.py for all APIs that have a generator.

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