Add $populate-missing

[ir4y]

It is not a part of the current SDC spec.

However, I found it useful to have an operation that will populate missing items to the existing QuestionnaireResponse.
It may be useful when the Questionnaire was updated, or to restore eliminated items after apply enableWhen.

The proposal is to add /QuestionnaireResponse/<id>/$populate-missing that will do a merge of existing QuestionanireResponse and populated version of it.