Add encounter extensions to link OpenMRS visits to encounters

OpenMRS has the concept of visits, which we use to group encounters.
For FHIR purposes, we use the FHIR Encounter resource to represent both OpenMRS Encounters and visits.
The FHIR Encounter resource has the "partOf" attribute, which is used to represent 0..1 Encounters that this encounter is part of.

If we're exporting an OpenMRS encounter, and this encounter is part of a visit, the partOf attribute is used to "point" to the visit.

On the other hand, if we're exporting an OpenMRS visit and the visit contains multiple OpenMRS encounters, we should use 1..* Encounter extensions to include links to these OpenMRS encounters