Feature gap: Add missed enable_flow_logs & state to subnetwork

[modular-magician]

This patch introduces new fields to the Terraform provider for the subnetwork resource:

• enable_flow_logs - a field that allows enabling flow logs. If the log_config field is not set in the configuration, the default log_config value returned by the API will be stored in tfstate.
• state - a read-only field. During testing, it was observed that the API does not return this field in response to a GET request. Nevertheless, the field is stored in tfstate as an empty string.

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compute: added `enable_flow_logs` and `state` fields to `google_compute_subnetwork` resource

Derived from GoogleCloudPlatform/magic-modules#13093