Extend RelatedPerson search api

Presently, the search api for related person only allows for a search by UUID of the relationship itself. For example (first login with user fhir password Fhir1234)  https://openmrs-spa.org/openmrs/ws/fhir/RelatedPerson/1bbb0215-bd61-42d6-be1e-29f8fc7a0fb1

. Here are docs to the fhir api for this resource (scroll down to bottom for search api): http://www.hl7.org/fhir/stu3/relatedperson.html

We need to extend this api to allow for search by:

identifier : the identifier of the patient (and not id/uuid) for which you are looking for relatives. e.g. https://openmrs-spa.org/openmrs/ws/fhir/RelatedPerson?identifier=100EHD