Apply ISP on ResourceFieldEnhancerInterface

(This is a follow-up on a previous Slack conversation.)

Problem/Motivation

Resource field enhancers are valuable additions by this module to JSONAPI, however, the Drupal\jsonapi_extras\Plugin\ResourceFieldEnhancerInterface interface is too big, and it requires too much responsibility from implementations. It depends on smaller interfaces/contracts, so it should be easy to explode and create dedicated plugin interfaces and managers for only transforming or untransforming (or both like now).

The PHP library behind the scenes → already has ISP implemented and it allows to perform one-way transformations (like only manipulating the input) instead of building data adaptors always (which has its own challenges, see 🌱 Refactor field enhancer schema generation Active ).

Steps to reproduce

Proposed resolution

Introduce dedicated plugins and plugin managers for manipulating input and output data (or both).

Remaining tasks

User interface changes

API changes

New APIs are gonna be introduced, the existing one probably gets deprecated.

Data model changes