Authenticate Bitbucket Packages with native Pipelines authentication

Introduces native authentication for Bitbucket Packages container registry using Bitbucket Pipelines' built-in `BITBUCKET_PACKAGES_USERNAME` and `BITBUCKET_PACKAGES_TOKEN` environment variables. This eliminates the need for manual token generation, storage, and rotation, enhancing security with short-lived, per-pipeline-step tokens. Provides examples for direct `docker login` and using the `atlassian/bbc-packages-push-container-image` pipe.