Add support for valueString Value Type Template

[mmacagno]

As title says.
Add additional property to the bag of supported value types.
Useful to transport attributes from messages to the FHIR observation without having to enumerate all possible values.

Example:
given the message:

{ "Body" : { "patientReliefStarted": "occurred", "reliefType": "forwardRelief" // multiple values possible } }

I want to project all the reliefType values in one string attribute together with the CodeableConcept "patientReliefStarted". Currently it's not possible to achieve.

Tracking issue, I will create a PR for this.