feat(internal/gengapic): move operations & iterators to aux.go (#1428)

Makefile

@@ -45,4 +45,4 @@ clean:
 	rm -f showcase/showcase_grpc_service_config.json
 	rm -f showcase/compliance_suite.json
 	rm -f showcase/showcase_v1beta1.yaml
-	cd showcase; go mod edit -dropreplace github.com/googleapis/gapic-showcase
+	git restore showcase/go.mod showcase/go.sum

internal/gengapic/BUILD.bazel

@@ -3,6 +3,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 go_library(
     name = "gengapic",
     srcs = [
+        "auxiliary.go",
         "client_init.go",
         "custom_operation.go",
         "doc_file.go",
@@ -48,6 +49,7 @@ go_library(
         "@org_golang_google_protobuf//proto",
         "@org_golang_google_protobuf//reflect/protodesc",
         "@org_golang_google_protobuf//runtime/protoiface",
+        "@org_golang_google_protobuf//reflect/protoreflect",
         "@org_golang_google_protobuf//types/descriptorpb",
         "@org_golang_google_protobuf//types/known/anypb",
         "@org_golang_google_protobuf//types/known/durationpb",
@@ -63,6 +65,7 @@ go_library(
 go_test(
     name = "gengapic_test",
     srcs = [
+        "auxiliary_test.go",
         "client_init_test.go",
         "custom_operation_test.go",
         "doc_file_test.go",