feat: add custom env var field to Jobs

[dayeol]

In order to support custom variables for TEE, we need to generalize the "environment variables" which will be passed to the Job. For example, we can do the following:

when we submit job, we submit a spec of the job like below:

job:
  jupyterfile: main.ipynb
  env:
    INPUT_FILE="a.input"
    OUTPUT_FILE="b.output"
  ...

These env vars are stored in the DB, and the TEE backend can supply these variables when it launches the CVM instance. In confidential space, these additional environment variables are needed to be specified in Dockerfile.

cc @xxxxavier @privacywill