Add possibility to handle array of values into single entity

Looks good. Just to make sure I understand everything, it means you would get a single entity with no text keys and you will have to map numeric "keys" to fields then, right?

Correct, i create a custom_field "Expertise Item field" that have a "firstname" and "lastname" field.
Then i can map then like : firstname => $.*.firstName
This will populate the "Expertise Item field" with multiple items with each have "firstname" and "lastname" filled in.

@arwillame, could you add a test in RestClientFunctionalTest.php to test your use case and ensure the fix remains stable in the future?
Your use case could be documented in the test to avoid regressions.