Spring-ai Refine delineation of Embedding properties from OpenAI (client) properties

This applies Spring Boot's style to the definition of @ConfigurationProperties classes (for example) to OpenAiProperties.

I am somewhat undecided about defining a proper, additional property namespace for the AI client, such as:

spring.ai.openai.client.*

For example:

spring.ai.openai.client.api-key.

Comment From: markpollack

I think having the additional scope after sping.ai.openai makes sense. Though I'm not sure 'client' will stick, as I'm thinking to rename AiClient to GenerateClient or TextGenerationClient. We need to go through other APIs that openai offers and think up the right segmentation of configuration to plan out for the future. Will address this during overall reviews.