feat(storagetransfer): add private_network_service to azure_blob_storage_data_source

[favoretti]

Summary

Exposes the privateNetworkService field on AzureBlobStorageData in google_storage_transfer_job, enabling transfers from Azure Blob Storage over a customer-managed VPC via Service Directory. The field is already present in the SDK; the provider just wasn't surfacing it.

Format: projects/{projectId}/locations/{location}/namespaces/{namespace}/services/{service}.

This is fully additive — the new attribute is Optional with no default and no perma-diff. Backward compatible with all existing configurations.

Note: the same privateNetworkService field exists on AwsS3Data (separate from the existing managed_private_network bool) and the provider also doesn't expose it there. Left out of this PR to keep it focused; happy to follow up if reviewers want them bundled.

Test plan

• New acceptance test added: TestAccStorageTransferJob_azureBlobStorageDataSourcePrivateNetwork
  • Creates a real Service Directory namespace + two services so the references resolve API-side
  • Uses a placeholder Azure storage account / container / SAS token (the STS API accepts unvalidated source specs at create time, mirroring the existing awsS3CompatibleDataSource test)
  • Asserts private_network_service round-trips through state on create
  • Update step switches private_network_service to a second Service Directory service to exercise mutability
  • Each apply is followed by an import step (ImportStateVerifyIgnore on azure_credentials since it's input-only per the API)
• Acceptance test execution pending — author does not have a test project with Azure / Service Directory set up for STS

Release Note Template for Downstream PRs (will be copied)

See Write release notes for guidance.

storagetransfer: added `private_network_service` to resource `google_storage_transfer_job`

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[favoretti]

@SirGitsalot Could you have a look please? Thank you!