client-go/features: introduce feature gates

This PR add a feature gates mechanisim to client-go
as described in https://docs.google.com/document/d/1g9BGCRw-7ucUxO6OtCWbb3lfzUGA_uU9178wLdXAIfs

In particular:
  - Adds a default feature gate implementation based on environment variables.
  - Adds a set of methods for reading, overwriting the default implementation, and adding features to an external registry.

Co-authored-by: deads2k <deads@redhat.com>
Co-authored-by: Ben Luddy <bluddy@redhat.com>

Kubernetes-commit: d74c57d4f592d20a992afb54b1ee64f56215210e

features/features_test.go

@@ -0,0 +1,40 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package features
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestAddFeaturesToExistingFeatureGates ensures that
+// the defaultKubernetesFeatureGates are added to a test feature gates registry.
+func TestAddFeaturesToExistingFeatureGates(t *testing.T) {
+	fakeFeatureGates := &fakeRegistry{}
+	require.NoError(t, AddFeaturesToExistingFeatureGates(fakeFeatureGates))
+	require.Equal(t, defaultKubernetesFeatureGates, fakeFeatureGates.specs)
+}
+
+type fakeRegistry struct {
+	specs map[Feature]FeatureSpec
+}
+
+func (f *fakeRegistry) Add(specs map[Feature]FeatureSpec) error {
+	f.specs = specs
+	return nil
+}