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/external_doc.go

@@ -0,0 +1,25 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package base
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	"github.com/pb33f/libopenapi/index"
+	"gopkg.in/yaml.v3"
+)
+
+type ExternalDoc struct {
+	Description low.NodeReference[string]
+	URL         low.NodeReference[string]
+	Extensions  map[low.KeyReference[string]]low.ValueReference[any]
+}
+
+func (ex *ExternalDoc) FindExtension(ext string) *low.ValueReference[any] {
+	return low.FindItemInMap[any](ext, ex.Extensions)
+}
+
+func (ex *ExternalDoc) Build(root *yaml.Node, idx *index.SpecIndex) error {
+	ex.Extensions = low.ExtractExtensions(root)
+	return nil
+}