Add merge request pipeline to hpc-factory .gitlab-ci.yml

Include merge_request_event in workflow.rules to enable merge request pipeline that can be trigger when new merge request created.

• refactor old pipeline at https://gitlab.rc.uab.edu/rc/hpc-factory/-/blob/2818a7762a1cf132146b046c2376880de1d5e908/.gitlab-ci.yml

Rules related to merge requests we might want to include:

• Skip pipelines for draft merge requests
• Rule to prevent duplicate pipelines
  • Switch between branch pipelines and merge request pipelines