Spring-ai Overload QuestionAnswerAdvisor to use SearchRequest.defaults()

Although it's helpful to be able to customize the SearchRequest given to QuestionAnswerAdvisor, it's not uncommon to just rely on SearchRequest.defaults(). Therefore, this PR overloads the constructor with a version that only requires a VectorStore and uses SearchRequest.defaults()...in the interest of simplifying the application code a little.

Comment From: tzolov

Makes perfect sense. Thanks @habuma