US-Core: Implement Operation $docref

[prb112]

Is your feature request related to a problem? Please describe. This operation is used to return all the references to documents related to a patient.

Describe the solution you'd like

• Implement Operation $docref
• Add Operation Tests
• Drive off of the Search API
• https://www.hl7.org/fhir/us/core/OperationDefinition-docref.html

Describe alternatives you've considered None

Additional context None

[lmsurpre]

some related conversation: https://chat.fhir.org/#narrow/stream/179166-implementers/topic/.24docref.20operation.20Q see especially the [suggested] JIRA on a potential change from CodeableConcept to Codings, and the comment about not enforcing the required binding to http://hl7.org/fhir/ValueSet/c80-doc-typecodes