Modular RAG: Query Augmentor

[ThomasVitale]

• Add new QueryAugmentor API, a component for augmenting a user query with contextual data.
• Implement ContextualQueryAugmentor that combines the content of each document and add it to the original user prompt, with support for the scenario where the context is empty.
• Extend RetrievalAugmentationAdvisor to use the new augmentation building block.
• Introduce utility to assist in validating arguments for prompt-related operations.

Relates to gh-#1603

It also makes possible the customization reported in #1294

[tzolov]

Thanks @ThomasVitale
Changes look good.

• It reminded me that we need to make the Builder methods consistent. Some use with prefix other don't. But it seems the Spring's convention is not to have prefix (Consistent design of Options builder classes #1592 (comment))
• After we merge this PR, we can refactor the QuedstionAnswerAdvisor as a wrapper around the RetrieverAugmentor one. Not sure if we should mark it as deprecated though.

[tzolov]

Rebased and merged at bc50d28

[ThomasVitale]

@tzolov thanks for the review! For the builder naming convention, I followed what was used already for ChatClient, but indeed both strategies are used throughout the project. It would be nice to converge towards one single naming strategy. I like the Spring convention of not having the prefix.

[ThomasVitale]

About the QuestionAnswerAdvisor, it's used a lot by developers. To protect the users from breaking changes, I would suggest keeping it as-is, mark it as deprecated, and document how to migrate to RetrievalAugmentationAdvisor. We could even provide a simple OpenRewrite recipe to automate the migration.

[tzolov]

Makes sense.
Have not used the OpenRewrite yet and curious to see how it looks like :)