Update the Flask app to use custom resource: `LMEvalJob`

[yhwang]

Currently, the Flask app runs the lm-eval job by spawning subprocesses within the same Pod. Since the CRD and controller is ready, we need to update the implementation to:

• submit_job API, create an LMEvalJob CR
• poll_job API, check the corresponding CR's status
• job_results API, retrieve the results from the corresponding CR
• list_jobs, get a list of LMEvalJob
• cancel_job, update the LMEvalJOb's status field to initiate the concealing flow