Add the computed shapes for keys that are not fields

Problem/Motivation

Part of every response is not a field at all: the alias, the label, the created and changed timestamps, the metatags, the available translations, the view and like counters, the reading time. They are computed from the entity or from the environment, and today they are assembled inline in each resource.

Proposed resolution

• Express each as a shape with a source of id, label, alias, created, changed, locales, metatags rather than a field name, so a structure describes them the same way it describes everything else.
• Take the entity keys from the entity type definition, so that the computed keys work for any content entity type.
• The ones that depend on a site-filled role — counters, reading time, metatags source — return nothing at all when the role is unfilled.
• Alias and locales must work for entity types whose canonical route is not /node/{id}.

Remaining tasks

Everything.