Setup GitLab based CI pipeline

Summary

This PR aims to migrate the current Jenkins based CI pipeline to GitLab CI. It tries to achieve exactly the same behaviour that was happening via the Jenkins pipeline. Specifically, do the following job

1. Build image (every new push)
2. Run test cases (every new push)
3. Push package to Test PyPI (if 1 & 2 passes AND it's a merge request)
4. Push package to PyPI (if 1 & 2 passes AND it's a tag)

2 new deployment environments have been created:

1. staging - used to push to Test PyPI
2. production - used to push to PyPI