Add: allow arbitrary output_fields when dynamic field is enabled

[DrChristophFH]

• Don't reject specified output fields that do not match the schema fields if the dynamic field is enabled

This allows to specify arbitrary metadata fields to be included in the output when building a retriever:

retriever = vectorstore.as_retriever(
    search_type="similarity",
    search_kwargs={
        "k": 8,
        "ranker_type": "weighted",
        "ranker_params": {
            "weights": [
                0.5,  # Dense
                0.5,  # Sparse            ],
        },
        "output_fields": [
            "source",
            "section_title",
            "text",
            "pk",
        ],
    },
)