[FHIR]: Questionnaire.subjectType may be more strictly checked

[pauljohanneskraft]

We may want to think about checking values as part of Questionnaire.subjectType more strictly, as FHIR actually defines a long, but fixed list of possible values.

types/fhir only specifies a string as type here, but technically, it should be a fixed set of strings... not sure, if we want to keep it as string or not.

Originally posted by @pauljohanneskraft in #19 (comment)