feat: add multiple-choice questions

### Additions

* added new `MultipleChoiceQuestionDetailsType` type
* added `MultipleChoiceQuestionDetailsType` to `QuestionDetails` union
* added new `QuestionnaireAnswerStatus` enum

### Changes

* field `isCorrect: Boolean` changed to `correct: QuestionnaireAnswerStatus!` on `QuestionnaireAnswerType`
* field `answer: String!` type changed to `[String!]!` on `QuestionnaireAnswerType`
* field `value: String!` type changed to `[String!]!` on `AnswerInput`
* field `value: String!` type changed to `[String!]!` on `AnswerType`

Closes #403