Move kubeconfig utility to a shared pkg

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

internal/kubeconfig/kubeconfig.go

@@ -0,0 +1,65 @@
+package kubeconfig
+
+import (
+	"io"
+
+	"github.com/pkg/errors"
+	"gopkg.in/yaml.v3"
+)
+
+type ReadWriteResetCloser interface {
+	io.ReadWriteCloser
+
+	// Reset truncates the file and seeks to the beginning of the file.
+	Reset() error
+}
+
+type Loader interface {
+	Load() (ReadWriteResetCloser, error)
+}
+
+type Kubeconfig struct {
+	loader Loader
+
+	f        ReadWriteResetCloser
+	rootNode *yaml.Node
+}
+
+func (k *Kubeconfig) WithLoader(l Loader) *Kubeconfig {
+	k.loader = l
+	return k
+}
+
+func (k *Kubeconfig) Close() error {
+	if k.f == nil {
+		return nil
+	}
+	return k.f.Close()
+}
+
+func (k *Kubeconfig) Parse() error {
+	f, err := k.loader.Load()
+	if err != nil {
+		return errors.Wrap(err, "failed to load")
+	}
+
+	k.f = f
+	var v yaml.Node
+	if err := yaml.NewDecoder(f).Decode(&v); err != nil {
+		return errors.Wrap(err, "failed to decode")
+	}
+	k.rootNode = v.Content[0]
+	if k.rootNode.Kind != yaml.MappingNode {
+		return errors.New("kubeconfig file is not a map document")
+	}
+	return nil
+}
+
+func (k *Kubeconfig) Save() error {
+	if err := k.f.Reset(); err != nil {
+		return errors.Wrap(err, "failed to reset file")
+	}
+	enc := yaml.NewEncoder(k.f)
+	enc.SetIndent(2)
+	return enc.Encode(k.rootNode)
+}