Added network and subnetwork fields to regional NEG resource

[modular-magician]

Added network and subnetwork fields to google_compute_region_network_endpoint_group for PSC. These are required when creating a PSC-type NEG that points to a service attachment.

Link to GA API: https://cloud.google.com/compute/docs/reference/rest/v1/regionNetworkEndpointGroups/insert

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Release Note Template for Downstream PRs (will be copied)

compute: Added `network` and `subnetwork` fields to `google_compute_region_network_endpoint_group` for PSC.

Derived from GoogleCloudPlatform/magic-modules#6275