feature: add user cluster create with terraform sample (manuallb)

[Shabirmean]

Fixes b/270587524

Description

• Adds a new sample for creating Anthos on bare metal user cluster with ManualLB using the Terraform provider.

Change summary

• Add new directory under anthos-onprem-terraform called abm_user_cluster_manuallb
• Add a new sample for creating a user cluster with manuallb

Related PRs/Issues

• N/A

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• anthos-onprem-terraform/abm_user_cluster_manuallb/main.tf:17, tag anthos_onprem_terraform_bare_metal_user_cluster_manuallb_main
• anthos-onprem-terraform/abm_user_cluster_manuallb/terraform.tfvars.sample:1, tag anthos_onprem_terraform_bare_metal_user_cluster_manuallb_tfvars

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[bourgeoisor]

│ Error: Failed to query available provider packages
│ 
│ Could not retrieve the list of available versions for provider
│ hashicorp/google-private: provider registry registry.terraform.io does not
│ have a provider named registry.terraform.io/hashicorp/google-private

[Shabirmean]

│ Error: Failed to query available provider packages
│ 
│ Could not retrieve the list of available versions for provider
│ hashicorp/google-private: provider registry registry.terraform.io does not
│ have a provider named registry.terraform.io/hashicorp/google-private

We will be seeing this error until the Terraform provider is public previewed next week. Currently testable only internally!

[Shabirmean]

@naitianliu-google - Can you TAL at this and confirm if we can merge this in?

[minherz]

lgtm