compute: added subnetwork_id to compute subnet data source

[wyardley]

Added subnetwork_id to google_compute_subnetwork data source

Picked from #12285 (comment here)
Followup to #12351
Related to hashicorp/terraform-provider-google#20223
Related to hashicorp/terraform-provider-google#20530

Release Note Template for Downstream PRs (will be copied)

compute: added `subnetwork_id` to `google_compute_subnetwork` data source

[github-actions]

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[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 3 files changed, 11 insertions(+))
google-beta provider: Diff ( 3 files changed, 11 insertions(+))

[modular-magician]

Tests analytics

Total tests: 1071
Passed tests: 998
Skipped tests: 73
Affected tests: 0

Click here to see the affected service packages

• compute

🟢 All tests passed!

View the build log

[github-actions]

@GoogleCloudPlatform/terraform-team @NickElliot This PR has been waiting for review for 1 week. Please take a look! Use the label disable-review-reminders to disable these notifications.

[NickElliot]

LGTM!