Enhance x-fhir-query fhirpath interpolation for multiple answers

[ruscoder]

https://github.com/beda-software/aidbox-sdc/blob/95e92ca7b84d38ebc3cf41898c4e6a32b5ded2f6/app/operations/utils.py#L95

http://build.fhir.org/ig/HL7/sdc/expressions.html#x-fhir-query-enhancements

If the expression resolves to a collection of more than one value, the substitution will be a list of comma-separated values (i.e. behaving as 'or').