Add devcontainer/Codespaces setup so beginners can run examples without worrying about setup

[jonathanagustin]

Adding a devcontainer/Codespaces setup will allow beginners to immediately use OpenAI's API without worrying about setting up their environment. The setup includes cloning the openai-cookbook repository and installing most of the dependencies required to run the examples. After building and setting the API key, a user can immediately start executing the examples.

With greater accessibility of infrastructure, more people can learn and use OpenAI's Python API. If this PR is merged, the README.md should include notice to users about potential Codespace development on the cloud without worrying about the environment.

I chose to scope the entirety of the PR to the .devcontainer folder to contain the entirety of the feature. I chose a Python3.8 image because some dependencies required it.