Support derived source for knn with other fields

[neetikasinghal]

Description

Implement derived source for k-NN fields with core implementation, with a fallback to k-NN plugin's implementation.

Related PR: opensearch-project/OpenSearch#20991

Before this change, if index.derived_source.enabled and there is a knn field type, there will be error thrown saying that knn vector field doesnt supprt derived source feature.
After this change, if index.derived_source.enabled and there is a knn field type, the derived source feature will be turned on so that users can benefit out of both knn fields and other fields.

Related Issues

Resolves #3058

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[Vikasht34]

I have one quick question Here , So Dervied Source is enabled at Index Time and can be mappedFieldType.hasDocValues() is false so DerivedFieldGenerator will choose stored path but storedFieldFetcher = null in this PR , will it not cause NullPointerException .

[neetikasinghal]

yes for that, i was planning to make a change like:

return new DerivedFieldGenerator(mappedFieldType, new KnnVectorValuesFetcher((KNNVectorFieldType) mappedFieldType, simpleName()), null) {
            @Override
            public FieldValueType getDerivedFieldPreference() {
                return FieldValueType.DOC_VALUES;
            }
        };

I was planning to use FieldValueType.DOC_VALUES always for knn case, KnnVectorValuesFetcher can be used as the FieldValueFetcher. Ideally we should have a new FieldValueType something like FieldValueType.KNN_VALUES, but since i dont see knn using DOC_VALUES, we can leverage FieldValueType.DOC_VALUES for vector values. what do u think?

[Vikasht34]

So I understand why we are skipping here because KNN is having Mapping Validation , correct me if I am wrong but how we will make sure if I excluded-from-source fields are never later considered derivable accidentally

[neetikasinghal]

in other mappers, canDeriveSourceInternal calls checkStoredAndDocValuesForDerivedSource but for knn field mapper it is not needed for either of the doc values and stored flag to return true, so dont see a need to put any checks here.

protected void checkStoredAndDocValuesForDerivedSource() {
        if (!mappedFieldType.isStored() && !mappedFieldType.hasDocValues()) {
            throw new UnsupportedOperationException("Unable to derive source for [" + name() + "] with stored and " + "docValues disabled");
        }
    }