[install] chore: add pyproject.toml. make vllm default dependency

[eric-haibin-lin]

Verified installation with official docker image:

Processing /root/code/verl
  Running command pip subprocess to install build dependencies
  Using pip 24.2 from /root/miniconda3/lib/python3.10/site-packages/pip (python 3.10)
  Looking in indexes: https://pypi.tuna.tsinghua.edu.cn/simple
  Collecting setuptools>=42
    Using cached https://pypi.tuna.tsinghua.edu.cn/packages/55/21/47d163f615df1d30c094f6c8bbb353619274edccf0327b185cc2493c2c33/setuptools-75.6.0-py3-none-any.whl (1.2 MB)
  Collecting wheel
    Using cached https://pypi.tuna.tsinghua.edu.cn/packages/0b/2c/87f3254fd8ffd29e4c02732eee68a83a1d3c346ae39bc6822dcbcb697f2b/wheel-0.45.1-py3-none-any.whl (72 kB)
  Installing collected packages: wheel, setuptools
    Creating /tmp/pip-build-env-og13ok4i/overlay/bin
    changing mode of /tmp/pip-build-env-og13ok4i/overlay/bin/wheel to 755
  Successfully installed setuptools-75.6.0 wheel-0.45.1
  WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager, possibly rendering your system unusable.It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv. Use the --root-user-action option if you know what you are doing and want to suppress this warning.
  Installing build dependencies ... done
.......