Add gitlab_project_integration_google_chat resource

What does this MR do?

Adds a new Terraform resource gitlab_project_integration_google_chat to manage the native Google Chat (Hangouts Chat) project integration via the GitLab API.

#6738 (closed) Feature request to support Google Chat project integration as a Terraform resource.

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Why is it needed?

Although the GitLab API supports the Google Chat integration endpoint (/projects/:id/integrations/hangouts-chat), there is currently no corresponding Terraform resource. This prevents users from managing this integration using infrastructure as code.

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What’s included?

• resource_gitlab_project_integration_google_chat.go
  Full CRUD implementation using:
  • SetProjectGoogleChatSettings
  • GetProjectGoogleChatSettings
  • DisableProjectGoogleChat
• resource_gitlab_project_integration_google_chat_test.go
  Acceptance tests covering create, update, and import scenarios

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Notes

• The webhook attribute is marked as sensitive and excluded from ImportStateVerify, since the API does not return it after creation.
• Users must reconfigure the webhook and run terraform apply after importing.
• Changing the project attribute forces resource recreation.
• This MR depends on upstream support added in gitlab-org/api/client-go!2222.