Support for patient parameter in Group export

[joanne-m]

Patient is an optional parameter for bulk data export. This issue adds a new filter for group export to include only patients within the provided reference list.

From https://build.fhir.org/ig/HL7/bulk-data/export.html

Query Parameter	Optionality for Server	Optionality for Client	Cardinality	Type	Description
patient(POST requests only)	optional	optional	0..*	FHIR Reference	Not applicable to system level export requests. When provided, the server SHALL NOT return resources in the patient compartments belonging to patients outside of this list.

Out of scope: If a client requests patients who are not present on the server (or in the case of a group level export, who are not members of the group), the server SHOULD return details via a FHIR OperationOutcome resource in an error response to the request.A server that is unable to support patient SHOULD return an error and FHIR OperationOutcome resource so the client can re-submit a request omitting the patient parameter. When a Prefer: handling=lenient header is included in the request, the server MAY process the request instead of returning an error.