add an API for prev/next entity

Problem/Motivation

I am thinking of adding support for Computed Fields module, and the first step would be to add methods to EntityPager for getting the prev/next entity.

This would mean a bit of internal reworking of the EntityPager class, as currently it's entirely geared to being used from within Views theming, which happens after the view has been executed, and it detects the current entity from the current route.

For the API, we'd pass in a View ID and display ID and current entity, and it would then execute the view (but not render it) and return the entity. That's a very different pathway!

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes