Fetching visits of many pets at once

...les/petclinic/api/application/CustomersServiceClient.java

...amples/petclinic/api/application/VisitsServiceClient.java

...ples/petclinic/api/boundary/web/ApiGatewayController.java

...ringframework/samples/petclinic/api/dto/OwnerDetails.java

...springframework/samples/petclinic/api/dto/PetDetails.java

...rg/springframework/samples/petclinic/api/dto/PetType.java

...ringframework/samples/petclinic/api/dto/VisitDetails.java

...org/springframework/samples/petclinic/api/dto/Visits.java

...springframework/samples/petclinic/visits/model/Visit.java

...ework/samples/petclinic/visits/model/VisitRepository.java

...framework/samples/petclinic/visits/web/VisitResource.java

...ework/samples/petclinic/visits/web/VisitResourceTest.java