Support automated Nesting of referenced entities

Problem/motivation

Issue 2871653 → adds support for nested objects which is great.
However, nothing works out of the box until you implement a custom Processor. It would be good to have a generic way to nest referenced/child entities.

Proposed resolution

Add a new NestedEntity processor which will:

1. Identify which Entity fields are are configured as Object types (in search_api)
2. Automatically update these values to index as an object containing all of the entities data

User interface changes
A new Search processor is available "Nested Entity processor"

API changes
None.