feat: support demo server

[kemingy]

• [ ] image size (need to develop [optional])
• [ ] number of environments (need to develop the global limit)
• [ ] CPU/GPU/Memory for each environment (need to develop the global maximum limit)
• [ ] destroy all the environment every midnight (maybe a cron job)
• [ ] collect idle environment aggressively (need to develop)
• related to https://github.com/tensorchord/envd/issues/1309

Originally posted by @kemingy in https://github.com/tensorchord/envd-server/issues/46#issuecomment-1367297066