[FhirResourceTree] Support of "contentReference" elementDefinition attribute

[BPierrick]

Description

Some elements can be defined several times in a snapshot. When this happens, the next elementDefinitions can refer to another one using the attribute contentReference.

We need to support this behavior in the DefinitionNode tree building algorithm.

Example

Check on the StructureDefinition of Vital Signs Observation profile.
There is a description of the BackboneElement Component (Observation.component.referenceRange), that contains an attribute ReferenceRange. This attribute has no definition but the contentReference value referencing the elementDefinition Observation.referenceRange, which is actually another BackboneElement.