Add document-level filtering/metadata support for query

[linhnguyengts]

Feature Request: Support querying/filtering by specific documents or document metadata

Current behavior
Right now, when calling rag.query(...), there is no way to restrict the retrieval to only certain documents (by document ID, file name, source, tags, or any custom metadata). All documents in the index are always considered during retrieval.

Desired behavior
I would like to be able to query only a subset of the indexed documents, for example:

response = rag.query(
"...",
doc_ids=...,
)