Add the Convertible interface. (#369)

This sketches the "Convertible" interface that @vagababov outlined in
last week's Serving API WG meeting.

See also: https://docs.google.com/document/d/1UFUfwIF4q4r6JVigom0jQivbcI8mFycWPc9z48yjSIc/edit#heading=h.9np2by1b65b

apis/interfaces.go

@@ -35,6 +35,16 @@ type Validatable interface {
 	Validate(context.Context) *FieldError
 }
 
+// Convertible indicates that a particular type supports conversions to/from
+// "higher" versions of the same type.
+type Convertible interface {
+	// ConvertUp up-converts the receiver into `to`.
+	ConvertUp(to Convertible) error
+
+	// ConvertDown down-converts from `from` into the receiver.
+	ConvertDown(from Convertible) error
+}
+
 // Immutable indicates that a particular type has fields that should
 // not change after creation.
 type Immutable interface {