VA appointment requests: create service scaffolding and data mapping code

[jbalboni]

Do not update the application to use this service code, outside of unit tests.

Current service: /vaos/appointment_requests (just VA ones, not CC) New resource: /Appointment

AC:

• [ ] Service for the new resource has been created, if it doesn't already exist
• [ ] Service code is written to call current API function for service
• [ ] Errors are mapped to OperationOutcome format
• [ ] Successful service response is mapped to FHIR format
• [ ] Any data that is needed but has no place in FHIR structure is put inside a legacyVAR property
• [ ] Unit tests are written for new service and mapping code
• [ ] Create tickets where you were not able to map to the FHIR structure and/or other outstanding needs/issues

[jbalboni]

Blocked until we understand how requested times map to FHIR: https://issues.mobilehealth.va.gov/browse/WMSACT-1153