Add merge request approval settings to Terraform provider

Description

The GitLab Terraform provider is missing support for merge request approval settings available at the instance level in GitLab.

Related Documentation

• Enable merge request approval settings for an instance

Expected Behavior

Users should be able to configure merge request approval settings via Terraform at the global/instance level, similar to other application settings.

Acceptance Criteria

• Add support for prevent_approvals_by_commit_authors setting to the Terraform provider
• Add support for additional merge request approval settings (e.g., prevent_approvals_by_users_who_add_commits, require_password_to_approve)
• Settings should be configurable at the global/instance level
• Documentation should be updated to reflect the new settings

Implementation Guide

• Follow the CONTRIBUTING.md guide for setting up your local development environment.
• Clone the community fork of this project.
• Add three new attributes to internal/provider/sdk/schema_gitlab_application_settings.go:
  • prevent_merge_requests_author_approval
  • prevent_merge_requests_committers_approval
  • disable_overriding_approvers_per_merge_request
• Follow the existing attribute disable_personal_access_tokens to see where they need to be added.
• Add or update an existing acceptance test in internal/provider/sdk/resource_gitlab_application_settings_test.go to set the new attributes.