[SCRS-2230] Configure source IP forwarding for GCP Service Extensions

[eliottness]

What does this PR do? What is the motivation?

Fixes SCRS-2230

Updates the GCP Service Extensions Terraform deployment guide to configure forward_attributes = ["source.ip"] on google_network_services_lb_traffic_extension with provider = google-beta and update the required providers configuration to use google-beta. This ensures the service extension receives the client IP address observed by the load balancer for App and API Protection client-IP attribution.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/eliottness/gcp-service-extension-source-ip/security/application_security/setup/gcp/service-extensions

[eliottness]

Follow-up correction in cffde6ee1c: the Terraform resource type remains google_network_services_lb_traffic_extension; the beta implementation is selected with provider = google-beta. The forward_attributes field was released in provider v7.39.0, so the required-provider constraint is now >= 7.39.0.

[jhgilbert]

[Automated message] This PR has merge conflicts caused by the recent docs repo reorg (files moved from the repo root into hugo/). A new PR with your commits translated to the correct paths has been opened: #38852

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