Added support for labels field in the google_compute_external_vpn_gateway

[modular-magician]

Added support for the labels field in the google_compute_external_vpn_gateway resource.
fixes hashicorp/terraform-provider-google#7798

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Release Note Template for Downstream PRs (will be copied)

compute: added the `labels` field to the `google_compute_external_vpn_gateway` resource

Derived from GoogleCloudPlatform/magic-modules#7053