JSON string is escaped when access via REST

Problem/Motivation

If you have a JSON field on an entity that is accessed via REST, the JSON string is treated like any other string and escaped. This leads to weird outcomes if you have a < symbol inside your JSON, Drupal will truncate everything after the <.

Steps to reproduce

1. Create a json_field on a entity
2. Create an entity with a json_field value that a < within it.
3. Access via cores EntityResource endpoints, e.g. /entity/node/article

Proposed resolution

Provide a custom normalizer.

Remaining tasks

Tests