Add gitlab_project_package_dependency_proxy resource
Implements GitLab Issue #6210 - adds support for managing project-level package dependency proxy settings for Maven packages.
Closes #6210
This resource uses the GraphQL mutation updateDependencyProxyPackagesSettings to configure:
- enabled: Enable/disable the dependency proxy
- maven_external_registry_url: External Maven registry URL
- maven_external_registry_username: Optional authentication username
- maven_external_registry_password: Optional authentication password
Note: This feature requires GitLab Premium or Ultimate.