Swagger 2.0 high level model going in now

Shifting a few high level models around that are also shared. now it's just a churn game to flesh the high level model and test it up.

datamodel/high/base/external_doc.go

@@ -0,0 +1,33 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package base
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/high"
+	low "github.com/pb33f/libopenapi/datamodel/low/base"
+)
+
+type ExternalDoc struct {
+	Description string
+	URL         string
+	Extensions  map[string]any
+	low         *low.ExternalDoc
+}
+
+func NewExternalDoc(extDoc *low.ExternalDoc) *ExternalDoc {
+	d := new(ExternalDoc)
+	d.low = extDoc
+	if !extDoc.Description.IsEmpty() {
+		d.Description = extDoc.Description.Value
+	}
+	if !extDoc.URL.IsEmpty() {
+		d.URL = extDoc.URL.Value
+	}
+	d.Extensions = high.ExtractExtensions(extDoc.Extensions)
+	return d
+}
+
+func (e *ExternalDoc) GoLow() *low.ExternalDoc {
+	return e.low
+}