REST views: Create a facility to pretty-print serialized data and use in Views UI preview

Problem/Motivation

JSON and XML encoded documents are a pain to read, especially when working with Views to assemble full entity output for a number of different entities. Even objects with relatively simple payloads have sufficient "boilerplate" bracketing to make it very difficult to evaluate the results of a View configuration without saving the view and using the results with the browser plugin or command-line tool of your choice.

Furthermore, non-RESTful development around serialization has a similar problem, and development/troubleshooting of new serializers ubiquitously relies on external tools to make sense of the output.

Proposed resolution

• Option A: Add a contextual option to "pretty-print" serializer output. Serialization done by the Views UI Preview simply sets this argument to TRUE.
• Option B: Take the serialized output and run it through a decode/re-encode process that applies the pretty printing. This opens up options for using more tools, perhaps leaving the mechanism more in the hands of contrib.

As a side-effect, this would facilitate something like a Devel Serialization module which could add preview tabs for entities to see what their serialized output would be in configured formats.

Remaining tasks

• Decide if Core should handle this
• Pick an implementation option
• Code

User interface changes

Views UI Preview becomes readable for REST export data.

API changes

As a dedicated "pretty-printing" service, there would be API additions.
As a contextual parameter for serializers, there would be an additional, minimally documented parameter and a higher impact on Serialization tests.

Data model changes

None.