Add L7 ILB fields to RegionBackendService

[modular-magician]

Original Author: @tysen

compute: added support for L7 ILB to google_compute_region_backend_service.

compute: the `backend.group` field is now required for `google_compute_region_backend_service`. Configurations without this would not have worked, so this isn't considered an API break.

compute: extended backend configuration options for `google_compute_region_backend_service` to include `backend.balancing_mode`, `backend.capacity_scaler`, `backend.max_connections`, `backend.max_connections_per_endpoint`, `backend.max_connections_per_instance`, `backend.max_rate`, `backend.max_rate_per_endpoint`, `backend.max_rate_per_instance`, and `backend.max_utilization`

compute: `load_balancing_scheme` for `google_compute_forwarding_rule` now accepts `INTERNAL_MANAGED` as a value.