Currently, QuerySettings are defined as:

  QuerySettings:
    description: >-
      Struct representing possible settings that may be applied to a given query.
    fields:
      indexHints:
        type:
          variant: [IndexHintSpec, array<IndexHintSpec>]
        optional: true
      queryEngineVersion:
        type: QueryEngineVersion
        optional: true

While specifying a variant provides additional flexibility to the user (they can either provide an array of index hint specs or a single object), this creates rather ugly code on the server side.

As part of this patch, we would refactor this (introduce custom serializer/deserializer methods), such that user can still specify IndexHintSpec or an array of specs, while we in code always have a vector of IndexHintSpecs.

The goal would be to define the serializer, deserializer methods as well as adding tests to ensure their correctness and adjust the breaking code, as now vector<IndexHintSpec> will be used.