core: allow artifact in create_retriever_tool (#28903)
Add option to return content and artifacts, to also be able to access
the full info of the retrieved documents.
They are returned as a list of dicts in the `artifacts` property if
parameter `response_format` is set to `"content_and_artifact"`.
Defaults to `"content"` to keep current behavior.
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Co-authored-by: Erick Friis <erick@langchain.dev>
