Set x-goog-user-project metadata from the quota_project config for all veneers

[dazuma]

Currently, the x-goog-user-project metadata is being set if the underlying credentials have a quota project. However, we want the option to override this via configuration. The quota_project config field has been reserved for this purpose.

Microgenerator-produced libraries already support this feature. We need to extend it to veneers.

This is a Q2 OKR for Yoshi.

• google-cloud-bigquery
• google-cloud-bigtable
• google-cloud-datastore
• google-cloud-debugger
• google-cloud-dns
• google-cloud-error_reporting
• google-cloud-firestore
• google-cloud-logging
• google-cloud-pubsub
• google-cloud-resource_manager
• google-cloud-spanner
• google-cloud-storage
• google-cloud-translate-v2 (?)

[quartzmo]

Related: https://cloud.google.com/sdk/gcloud/reference/auth/application-default/set-quota-project