Added primaryCarePatientIdType global property + Proposed method of handling patient identifier types

PatientIdentifier Types are used and handled similarly in the following
files:

EncounterServiceAdvice
ClientRegistryService
EnteredHandler
ScheduledHandler
SharedHealthRecordService
HL7MessageTransformer

In each case, the Primary Care ID Type is handled differently, while
all others are managed in the same way.

Here I propose how to handle this case in EncounterServiceAdvice. If
this method is adopted, I will make similar changes to the other files
using PatientIdentifier Types, or consider using a util method to
manage this.