[Feature Request] RAG citation and function call

[Wendong-Fan]

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Motivation

we need to be able get citation for the content created with RAG, this part can be well handled with current metadata output from retriever
we need to add retrieval function for user/agent to easily call

Solution

ensure quality from unstructured io meta data
add retrieval function under camel functions

Alternatives

No response

Additional context

No response