Add support for Atomic Operations Extension

_Atomic Operations Extension_ (json-api/json-api#1437) and the somewhat related _Local Identities Extention_ : json-api/json-api#1436) are the two cornerstones of the path taken by JSON:API spec to support multiple operations in HTTP one request.

The work done in #282 can hopefully be reused and adapted. I'm gonna work on it once I have a working server-side implementation for Drupal.