Enhancement: Support prop to filter results by a particular index

**Issue Type:**

Enhancement

**Platform:**

All

**Description:**

When ReactiveSearch is used against multiple indices / cluster, there can an additional `index` (or `appFilter`, name can be decided) prop within each component that allows specifying which index/index pattern should be used to query against for the component.

Internally, this would work as mentioned in #545. Currently, this behavior is possible by the user specifying a `customQuery`. Having a prop present for it would make this possible out of the box.

**Screenshots:**

N/A

**Minimal reproduction of the problem with instructions:**

**Reactivesearch version:** x.y.z

2.13.0, i.e. latest