openai: add support for setting the base url

This commit adds support for setting the base url for the openai client. This is useful for testing and for using the client with a different provider.

The motivation for this came from wanting to try out Perplexities API using llm-chain-openai. For this I needed to set the base url to the one provided by Perplexity, but I could find a way to that with the current implementation.

With the change in this commit, an example can be written like this:

use llm_chain::options;
use llm_chain::options::ModelRef;
use llm_chain::{executor, parameters, prompt};

async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let opts = options!(
         Model: ModelRef::from_model_name("pplx-70b-online")
    );
    let exec = executor!(chatgpt, opts.clone())?;
    let query = "What is the capital of Sweden?";
    println!("Query: {query}\n");
    let res = prompt!("", query,).run(&parameters!(), &exec).await?;
    println!("Perplixity AI:\n{res}");
    Ok(())
}

And the following environment variables need to be set:

export OPENAI_API_KEY=<Perplexity-API-Key>
export OPENAI_API_BASE_URL=https://api.perplexity.ai

sobelio / llm-chain

openai: add support for setting the base url #267