Specify input data type for CustomParticipantAttribute instead of generating one

[Whathecode]

When creating a CustomParticipantAttribute, an input data type is generated since the assumption was there is no benefit to specifying it yourself. The caller would have to guarantee uniqueness, so instead uniqueness is guaranteed by generating a GUID. In addition, it protects type namespaces.

... the Kotlin API currently doesn't allow to specify the InputDataType for CustomParticipantAttribute; it generates a unique ID in the format of dk.cachet.carp.input.custom.9004a473ce624c49bcec34a238174118. This is by design to protect the dk.cachet.carp.input namespace, but maybe is overly strict. The main goal here is to not pollute dk.cachet.carp.input with types for which support may be added later on, which would cause conflicts when comparing/handling data. E.g., if you use dk.cachet.carp.ssn now, but support gets added at with a stricter/different format. I think a suitable compromise is to allow you to set your own InputDataType, but restricting access to dk.cachet.carp sub-namespaces.

There was also an assumption knowing the type ID is irrelevant, since you wouldn't want to show it; instead, you would want to show the prompt of the matching InputElement. However, for extending study platforms which hardcode particular custom types, as opposed to them being created by the end user/researcher, setting a custom ID could help identify it in case modifications are needed, such as rendering a custom input form.

And, while the CustomParticipantAttribute doesn't allow setting a custom input data type, nothing prevents you from doing so in JSON payloads when calling CARP endpoints.

All in all, it does seem to make sense to allow users in the API to set their own input type id. In regards to protecting namespaces, consider this may be out of scope and a centralized solution may be preferred: #461.