Completed adding v3 models (I think)

Going to start logic for building things out. attempting generic builder pattern first.

datamodel/low/3.0/xml.go

@@ -0,0 +1,16 @@
+package v3
+
+import (
+    "github.com/pb33f/libopenapi/datamodel/low"
+    "gopkg.in/yaml.v3"
+)
+
+type XML struct {
+    Node       *yaml.Node
+    Name       low.NodeReference[string]
+    Namespace  low.NodeReference[string]
+    Prefix     low.NodeReference[string]
+    Attribute  low.NodeReference[string]
+    Wrapped    low.NodeReference[bool]
+    Extensions map[string]low.ObjectReference
+}