Add support for using GCP's Google Cloud Storage for storing archives #689
Description
When migrating away from GitHub Enterprise Server, a blob storage provider is used to store the archives from the Enterprise Server instance.
At the moment, we only support Azure Blob Storage and Amazon S3 for sharing the archives.
Some customers use Google Cloud Platform (GCP) as their chosen cloud provider, and may find it difficult to use another cloud provider just for this. We should add support for Google Cloud Storage in the CLI for blob storage.