Support Questionnaire launchContext for validation

[lmckenzi]

Logic in a Questionnaire (such as what elements are enabled) can be dependent on launchContext values - information passed into the form filler such as what the current patient, user, encounter, etc. are. Because these values can impact validation results, it would be helpful to be able to pass in values for launch contexts required by a Questionnaire as part of the validation process for a QuestionnaireResponse.