Search API: Support mapping/indexing referenced entity properties

Problem/Motivation

Field types of entity_reference, file and image have no ability to be mapped in Search API and therefore their properties cannot be indexed.

Proposed resolution

Add computed properties to these field types to return an entity in the data type class Search API can use to extract the properties.

Remaining tasks

Provide a patch for testing.

User interface changes

An expandable tree of properties will be present in the Search API for these field types.

Data model changes

A new computed property in the custom field types applicable to this issue.