Add internal ipv6 prefix into compute network and subnetwork data source reference attribute

[modular-magician]

… datasource

This PR is to make data_source_google_compute_network and data_source_google_compute_subnetwork include internal_ipv6_range and _internal_ipv6_prefix into their attribute reference respectively when they have dual stack support enabled upon creation.
Added field names are based on REST API reference and compute client protobuf:

• https://cloud.google.com/compute/docs/reference/rest/v1/networks
• https://cloud.google.com/compute/docs/reference/rest/v1/subnetworks
• https://pkg.go.dev/google.golang.org/api/compute/v1#Network
• https://pkg.go.dev/google.golang.org/api/compute/v1#Subnetwork

Release Note Template for Downstream PRs (will be copied)

compute: added `internal_ipv6_range` to `data.google_compute_network` data source and `internal_ipv6_prefix` field to `data.google_compute_subnetwork` data source

Derived from GoogleCloudPlatform/magic-modules#9274

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