[PH-5] Add support for user-defined environment variables

[raymondnguyen8]

PR checklist

• [x] Ensure you have added or ran the appropriate tests for your PR.
• [x] DCO signed

What type of PR is this? enhancement What this PR does / why we need it:

• [x] Define environment variables for the user jupyter notebook pod before starting the notebook server
• [x] Defined environment variables is automatically set after starting the notebook server and can be checked from the notebook shell after starting the notebook server
• [x] (Research) Keep the last setting of the environment variables of each user on the Select your notebook settings page after stopping the jupyter notebook pod for user-friendly reason
• [x] (Research) Environment variables setting should be only visible to the user itself and cannot be viewed by other users. Which issue(s) this PR fixes: https://github.com/orgs/InfuseAI/projects/5/views/1?pane=issue&itemId=44043793