Spring-ai Enable base URL to be configurable for OpenAI

Spring AI currently is fixed on using the official OpenAI URL when working with OpenAI. This makes it impossible to use Spring AI against a locally running OpenAI API such as LocalAI.

This PR exposes a new property (spring.ai.openai.base-url) that allows for custom configuration of the base URL. It still defaults to the official OpenAI API, though.

For example, to use this with a locally running LocalAI listening on port 8080, the following entry can be added to the application.properties file:

spring.ai.openai.base-url=http://localhost:8080/

Comment From: markpollack

I have merged this, just adding a comment that the underlying library will likely add this support as they have an open issue for it. I also enforced the use of the API KEY only if the baseUrl is pointing to api.openai.com

commit; 02180696133f644a2f1c3136458210e75a4c5a52

Thanks for the contribution.