This will introduce a new aggregation stage $scoreFusion

Description of Linked Ticket

Summary

Support result scoring based on combined results obtained from multiple search, vector-search and possibly non-search pipelines. 

This project is follow-up work to SPM-3541, specifically aimed at allowing users to define custom score composition functions.

Motivation

Users that have multiple search indexes (e.g. a regular search index and vector search index) may want to retrieve documents based on a score that depends on both vector distance and match quality to a regular search query.

We want to provide a reasonably efficient and convenient way of obtaining a combining results from multiple pipelines that return documents with their respective scores. 

Documentation

Product Description
Scope
Technical Design
Docs Update