Do not automatically decode runtime.RawExtension

Make clients opt in to decoding objects that are stored
in the generic api.List object by invoking runtime.DecodeList()
with a set of schemes. Makes it easier to handle unknown
schema objects because decoding is in the control of the code.

Add runtime.Unstructured, which is a simple in memory
representation of an external object.

pkg/conversion/error.go

@@ -54,6 +54,10 @@ type missingKindErr struct {
 	data string
 }
 
+func NewMissingKindErr(data string) error {
+	return &missingKindErr{data}
+}
+
 func (k *missingKindErr) Error() string {
 	return fmt.Sprintf("Object 'Kind' is missing in '%s'", k.data)
 }
@@ -70,6 +74,10 @@ type missingVersionErr struct {
 	data string
 }
 
+func NewMissingVersionErr(data string) error {
+	return &missingVersionErr{data}
+}
+
 func (k *missingVersionErr) Error() string {
 	return fmt.Sprintf("Object 'apiVersion' is missing in '%s'", k.data)
 }

pkg/conversion/scheme.go

@@ -238,6 +238,17 @@ func (s *Scheme) AddDefaultingFuncs(defaultingFuncs ...interface{}) error {
 	return nil
 }
 
+// Recognizes returns true if the scheme is able to handle the provided version and kind
+// of an object.
+func (s *Scheme) Recognizes(version, kind string) bool {
+	m, ok := s.versionMap[version]
+	if !ok {
+		return false
+	}
+	_, ok = m[kind]
+	return ok
+}
+
 // RegisterInputDefaults sets the provided field mapping function and field matching
 // as the defaults for the provided input type.  The fn may be nil, in which case no
 // mapping will happen by default. Use this method to register a mechanism for handling