OpenAiApi object is not registered as a bean via AutoConfiguration

[sivaprasadreddy]

Bug description
According to the Spring AI documentation (https://docs.spring.io/spring-ai/reference/api/chatclient.html#_multiple_openai_compatible_api_endpoints),
we can inject OpenAiApi and mutate to configure multiple OpenAI compatible ChatClients.
But it seems OpenAiApi object is not registered as a bean via AutoConfiguration.

Environment
Spring AI: 1.0.0, 1.1.0-SNAPSHOT.

Steps to reproduce
Try to inject OpenAiApi into any Spring bean.

Expected behavior
A bean of type OpenAiApi should be injected.

[sunyuhan1998]

Indeed, currently we've only registered the bean related to ChatModel. Although I'm not sure whether this is an oversight in the documentation, I think it might also be a good idea to automatically register the OpenAiApi bean.

Additionally, I noticed some inconsistencies in how we register XXApi beans across different model auto-configuration classes. For certain models, the auto-configuration class explicitly registers the XXApi as a bean, whereas for others, the corresponding XXApi is only created inline when registering the ChatModel bean, without being registered as a standalone bean itself.

@ilayaperumalg what's your opinion on this? If adjustments are needed, I'd be happy to contribute a PR.