Add Views data definition to join to Search API index

Problem/Motivation

It would be useful if it would be possible to join the book data to the search_api index so that book data could be displayed in a view of the search data.

Proposed resolution

Use hook_views_data_alter() to add the necessary relationship.

Remaining tasks

Use hook_views_data_alter() to add the necessary relationship.

User interface changes

Book data could be to a view that indexes node data.

API changes

TBD

Data model changes

TBD