Add benchmarks to measure impact of swagger parsing

Example results:

BenchmarkSwaggerParse-72               2         882910241 ns/op
BenchmarkAsssetUnpack-72              62          19654866 ns/op

kyaml/openapi/openapi_benchmark_test.go

@@ -0,0 +1,47 @@
+// Copyright 2021 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package openapi
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/go-openapi/spec"
+	"sigs.k8s.io/kustomize/kyaml/openapi/kubernetesapi"
+)
+
+// Benchmark for swagger parsing (UnmarshalJSON)
+func BenchmarkSwaggerUnmarshalJSON(t *testing.B) {
+	version := kubernetesOpenAPIDefaultVersion
+
+	// parse the swagger, this should never fail
+	assetName := filepath.Join(
+		"kubernetesapi",
+		version,
+		"swagger.json")
+
+	b := kubernetesapi.OpenAPIMustAsset[version](assetName)
+
+	for i := 0; i < t.N; i++ {
+		var swagger spec.Swagger
+		if err := swagger.UnmarshalJSON(b); err != nil {
+			t.Fatalf("swagger.UnmarshalJSON failed: %v", err)
+		}
+	}
+}
+
+// Benchmark for loading assets packed into the binary
+func BenchmarkAssetRead(t *testing.B) {
+	for i := 0; i < t.N; i++ {
+		version := kubernetesOpenAPIDefaultVersion
+
+		// parse the swagger, this should never fail
+		assetName := filepath.Join(
+			"kubernetesapi",
+			version,
+			"swagger.json")
+
+		kubernetesapi.OpenAPIMustAsset[version](assetName)
+	}
+}