Implement the FHIRPath repeat function

[lmsurpre]

Is your feature request related to a problem? Please describe.
While adding support for US Core 5.0, I found that their QuestionnaireResponse profile inherits some complex FHIRPath constraints from http://hl7.org/fhir/uv/sdc/STU3/StructureDefinition-sdc-questionnaireresponse.html

One of those constraints, sdcqr-2, uses the FHIRPath 'repeat' function which is not supported in our FHIRPath evaluator.

Describe the solution you'd like
Add support for repeat as defined in the FHIRPath specification: https://hl7.org/fhirpath/#repeatprojection-expression-collection

Describe alternatives you've considered
disable or override this constraint with a custom java implementation

Acceptance Criteria

1. GIVEN a QuestionnaireResponse that conforms to http://hl7.org/fhir/us/core/StructureDefinition/us-core-questionnaireresponse
   WHEN validated against http://hl7.org/fhir/us/core/StructureDefinition/us-core-questionnaireresponse
   THEN validation should complete with no exceptions

Additional context

[punktilious]

I downloaded all the JSON example resources from http://hl7.org/fhir/us/core/StructureDefinition-us-core-questionnaireresponse.html and was able to load them successfully using the latest fhir-is-us-core jar:

Examples:
# QuestionnaireResponse-AHC-HRSN-screening-example.json
# QuestionnaireResponse-glascow-coma-score.json
# QuestionnaireResponse-hunger-vital-sign-example.json
# QuestionnaireResponse-PRAPARE-example.json


Results:

HTTP/2 201
location: https://localhost:9443/fhir-server/api/v4/QuestionnaireResponse/AHC-HRSN-screening-example/_history/1
etag: W/"1"
last-modified: Thu, 11 Aug 2022 10:30:39 GMT
content-type: application/fhir+json
date: Thu, 11 Aug 2022 10:30:40 GMT
content-language: en-GB
content-length: 488

{"resourceType":"OperationOutcome","issue":[{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-name' is not supported"},"expression":["QuestionnaireResponse.meta.extension[0]"]},{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-description' is not supported"},"expression":["QuestionnaireResponse.meta.extension[1]"]}]}


HTTP/2 201
location: https://localhost:9443/fhir-server/api/v4/QuestionnaireResponse/glascow-coma-score/_history/1
etag: W/"1"
last-modified: Thu, 11 Aug 2022 10:34:23 GMT
content-type: application/fhir+json
date: Thu, 11 Aug 2022 10:34:23 GMT
content-language: en-GB
content-length: 488

{"resourceType":"OperationOutcome","issue":[{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-name' is not supported"},"expression":["QuestionnaireResponse.meta.extension[0]"]},{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-description' is not supported"},"expression":["QuestionnaireResponse.meta.extension[1]"]}]}


HTTP/2 201
location: https://localhost:9443/fhir-server/api/v4/QuestionnaireResponse/hunger-vital-sign-example/_history/1
etag: W/"1"
last-modified: Thu, 11 Aug 2022 10:35:19 GMT
content-type: application/fhir+json
date: Thu, 11 Aug 2022 10:35:19 GMT
content-language: en-GB
content-length: 488

{"resourceType":"OperationOutcome","issue":[{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-name' is not supported"},"expression":["QuestionnaireResponse.meta.extension[0]"]},{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-description' is not supported"},"expression":["QuestionnaireResponse.meta.extension[1]"]}]}


HTTP/2 201
location: https://localhost:9443/fhir-server/api/v4/QuestionnaireResponse/PRAPARE-example/_history/1
etag: W/"1"
last-modified: Thu, 11 Aug 2022 10:36:09 GMT
content-type: application/fhir+json
date: Thu, 11 Aug 2022 10:36:09 GMT
content-language: en-GB
content-length: 488

{"resourceType":"OperationOutcome","issue":[{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-name' is not supported"},"expression":["QuestionnaireResponse.meta.extension[0]"]},{"severity":"warning","code":"not-supported","details":{"text":"Extension definition 'http://hl7.org/fhir/StructureDefinition/instance-description' is not supported"},"expression":["QuestionnaireResponse.meta.extension[1]"]}]}