add optional service account for scheduled queries

[modular-magician]

• https://cloud.google.com/bigquery/docs/release-notes#November_21_2019
• see Scheduling queries, parameter documented at Try API or Discovery
• the API does support PATCHing, but magic-modules doesn't really support setting
  the required update_mask for url_param_only, hence I marked this parameter as
  input for now (which will replace the resource on update)
• followup to transport replaceVars url escape values #3347 (sorry for opening a new PR I renamed the branch)
  changes requested by slevenick are addressed and I ran terraform's acceptance test (TestAccBigqueryDataTransferConfig) and tested this manually w/ terraform

Release Note Template for Downstream PRs (will be copied)

* bigquery: added `service_account_name` field to `google_bigquery_data_transfer_config` resource

Derived from GoogleCloudPlatform/magic-modules#3405

[ghost]

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