Added generated_id field in the google_compute_backend_service to get the value of id defined by the server

[modular-magician]

Added support for the generated_id field in the google_compute_backend_service resource and datasource. This generated_id is mapped to the id field mentioned in the API docs.
fixes hashicorp/terraform-provider-google#2926

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compute: added support for `generated_id` field in `google_compute_backend_service` to get the value of `id` defined by the server

Derived from GoogleCloudPlatform/magic-modules#6950