Consolidate retry config for OpenAI

[ThomasVitale]

Extend the retry configuration strategy introduced in #1040 to the rest of the OpenAI model integration.

Main aspects:

• A RetryTemplate can be autoconfigured for each model type integration, or else a default one is used.
• The RetryTemplate is used to wrap the call to the model service for targeted HTTP retry logic instead of wrapping the entire implementation logic.
• The model request object is assembled in a dedicated method for improved readability and for sharing it between standard and streaming implementations.

[tzolov]

Thanks @ThomasVitale