Add SHARED_LOADBALANCER_VIP purpose to regional addresses.

[modular-magician]

Fixes: hashicorp/terraform-provider-google#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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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