Allow Nouveau index creation using the erlang query server

[jonasplaum]

Summary

Enable the use of the nouveau index function inside the native erlang query server.

Desired Behavior

The Index function like in javascript ( index("double", "min_length", doc.min_length, {"store": true}); ) should be supported in erlang. Just like the Emit(<<K>>, V) function, that's used for views.

Additional context

We are using one design doc with multiple indices per feature / service. All indices are using the native erlang query server. We want to keep the indices together, but the language is set per doc, which stops us from sharing one design doc for views and nouveau indices.