feat: Add .gitlab-ci.yml pipeline

[brendanator]

No description provided.

[github-actions]

Sourcery Review:

PR Type: Enhancement

PR Summary: This pull request introduces a new .gitlab-ci.yml file to set up a CI pipeline for automated code reviews using the Sourcery Coding Assistant. The pipeline is configured to run on merge request events and uses a Python slim Docker image to execute the necessary steps for installing Sourcery and performing the code review.

General suggestions:

• Ensure that all sensitive tokens such as SOURCERY_TOKEN and SOURCERY_AI_BOT are stored securely and are not logged or exposed in any way to maintain the security of the CI/CD pipeline.
• Consider pinning the version of sourcery-nightly to a specific version to avoid potential inconsistencies and incompatibilities that could arise from using a non-stable version of the tool.
• Review the CI pipeline configuration to ensure that it aligns with the project's requirements for code review and that it integrates smoothly with the existing CI/CD processes.