rebrand OPENAI_API_KEY to something generic if using litellm models

[lucasliet]

i've made this PR #575 to make it work with Gemini models
it turns out, it was already working on main branch, but you have to set the gemini api key as "OPENAI_API_KEY" to make it work, which could lead to confusion

maybe the key could be more generic, or have more than one key
and use OPENAI_API_KEY only if litellm is not enable, otherwise use the generic one

and reference it on README.md usage

also had to inject google-generativeai
pipx inject shell_gpt google-generativeai