allow use of OPENAI_API_KEY env var

[zzstoatzz]

AttributeError: 'NoneType' object has no attribute 'get_secret_value'

we fail if the api key:

• is not explicitly passed into a new client instance
• is not set as MARVIN_OPENAI_API_KEY in ~/.marvin/.env

most users will likely just have OPENAI_API_KEY set somewhere on their machine, so we should use that for marvin.settings.openai.api_key if they haven't done one of the above

this PR also updates the quickstart to recommend setting the env var instead of passing the client around everywhere