api_base should be read from env, same as api_key does

[AntoineRqe]

When using this crate, I wanted to specify a openai_api_base and I add to do

 let mut openai_config = OpenAIConfig::default();
    openai_config = openai_config.with_api_base(std::env::var("OPENAI_API_BASE").expect("OPENAI_API_BASE not set"));

Could be interesting to be able to read api_base from env variable in the constructor (same as openai_api_key).

I have the code ready for a pull request.

[64bit]

env var for base url is good to add, however the name of the env var should match what's officially used OPENAI_BASE_URL

https://github.com/search?q=repo%3Aopenai%2Fopenai-python%20OPENAI_BASE_URL&type=code

Pr is welcome!

[AntoineRqe]

Here is the PR
dd571b7

Need to check if request_options.rs:line 68 needs to be changed as well.