Refactoring core structure of datamodel to accomodate v2 model.

There is a lot of overlap on these models, to create a much more sensible design, a new 'base' package has been added which contains shared models that both versions make use of.

datamodel/low/base/xml.go

@@ -0,0 +1,20 @@
+package base
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	"gopkg.in/yaml.v3"
+)
+
+type XML struct {
+	Name       low.NodeReference[string]
+	Namespace  low.NodeReference[string]
+	Prefix     low.NodeReference[string]
+	Attribute  low.NodeReference[bool]
+	Wrapped    low.NodeReference[bool]
+	Extensions map[low.KeyReference[string]]low.ValueReference[any]
+}
+
+func (x *XML) Build(root *yaml.Node) error {
+	x.Extensions = low.ExtractExtensions(root)
+	return nil
+}