GCP Storage Sitemapper.Store implementation

You'll need to include the google_api_storage dependency to use this.

Configuration

• :bucket (required) -- a bucket to persist to
• :conn -- pass in your own GoogleApi.Storage.V1.Connection, depending on how you authenticate with GCP
• :path -- a path which is prefixed to the filenames
• :cache_control -- an explicit Cache-Control header for the persisted files