Gymlib + submodule infra

[wangpatrick57]

Summary: Set up submodules for agents and gymlib.

Demo:
The hello-tune agent (repo@614687b) calls print(gymlib.magic.get_magic_number()) which successfully prints 42.

Details:

• Each agent will be a git submodule inside agents/.
• Each agent will have access to gymlib/ as a Python library.
• Each agent must use its own conda environment. You can use .python_version and requirements.txt inside the agent to define the env. This can be created automatically using build_agent_conda_env.sh.
• The base gym will use the dbgym conda env. This can be created automatically using build_gym_conda_env.sh.
• The current tune/ folder is being deprecated and will be removed in a future PR. This is why the E2E tests are turned off in the CI.