Add SHARED_LOADBALANCER_VIP purpose to regional addresses.

[modular-magician]

Fixes: #7652

The docs are all over the place.

gcloud compute addresses create foo --purpose SHARED_LOADBALANCER_VIP --project REDACTED --region europe-west6 --subnet default --log-http works so I have made all 3 available at GA level.

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: added SHARED_LOADBALANCER_VIP as a valid option for `google_compute_address.purpose`

Derived from GoogleCloudPlatform/magic-modules#4169

[ghost]

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