Generate library for `gsuiteaddons.googleapis.com`

Despite the name, this is a service in Google Cloud Platform. It allows deployments to GCP of "add ons" for things like GMail and Google Docs.

Protos published at:

https://github.com/googleapis/googleapis/tree/master/google/cloud/gsuiteaddons/v1

The service is GA.

It may be complicated because it requires protos from an uncommon directory:

https://github.com/googleapis/googleapis/blob/9ce3738a6e6566ec6a919120c3f6bef13687e22b/google/cloud/gsuiteaddons/v1/gsuiteaddons.proto#L23-L29

The service overview is also at an unusual location:

https://developers.google.com/workspace/add-ons/overview

Internally these documents may help:

[go/add-ons-on-gcp:api](https://goto.google.com/add-ons-on-gcp:api)

[go/add-ons-on-gcf](https://goto.google.com/add-ons-on-gcf)