Update net-lb-app-ext security_settings variables

[wenzizone]

• set client_tls_policy as optional
• set subject_alt_names as optional

GitHub PR Description:

I am following the Medium article Private GCS Bucket Access Through Google Cloud CDN to create a Google Cloud CDN service using Terraform, with the backend being a private GCS bucket. However, I encountered an issue where the variables-backend-service.tf file requires the security_settings attribute to include both client_tls_policy and subject_alt_names.

If I set these two parameters, I receive the following error:

2024-12-23T10:20:23.655Z [DEBUG] provider.terraform-provider-google-beta_v6.14.1_x5:   "error": {
2024-12-23T10:20:23.655Z [DEBUG] provider.terraform-provider-google-beta_v6.14.1_x5:     "code": 400,
2024-12-23T10:20:23.655Z [DEBUG] provider.terraform-provider-google-beta_v6.14.1_x5:     "message": "Invalid value for field 'resource.securitySettings': '{  \"clientTlsPolicy\": \"tls-1-2\",  \"subjectAltNames\": [\"dev1.conviva.com\"],  \"awsV4Authentication\": {...'. Security settings is not supported."

However, according to the Google Cloud Compute Engine API documentationhere, clientTlsPolicy only applies to a global BackendService with the loadBalancingScheme set to INTERNAL_SELF_MANAGED. But the created GCS backend is for global internet use, so this parameter should not be required but optional. Similarly, subjectAltNames should also be optional as per the Google Cloud Compute Engine API documentation. This PR aims to address this issue by making client_tls_policy and subject_alt_names optional parameters in the google-beta provider, aligning with the actual usage and requirements of Google Cloud services. For reference, the client_tls_policy and subject_alt_names parameters are also optional in the google-beta provider as documented here.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• [] Made sure all relevant tests pass

Sorry, i don't know how to test, but with this change i success create GLB/ENG

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