Added mutate command to low level API

This simple method gives the low API a super powerful and simple way to mutate the value of any node, which is then reflected in the root node, can than be serialized again and, voila! now our spec is editable.

document.go

@@ -10,6 +10,8 @@ import (
 	v3high "github.com/pb33f/libopenapi/datamodel/high/3.0"
 	v2low "github.com/pb33f/libopenapi/datamodel/low/2.0"
 	v3low "github.com/pb33f/libopenapi/datamodel/low/3.0"
+	"github.com/pb33f/libopenapi/utils"
+	"gopkg.in/yaml.v3"
 )
 
 type Document struct {
@@ -40,6 +42,18 @@ func (d *Document) GetSpecInfo() *datamodel.SpecInfo {
 	return d.info
 }
 
+func (d *Document) Serialize() ([]byte, error) {
+	if d.info == nil {
+		return nil, fmt.Errorf("unable to serialize, document has not yet been initialized")
+	}
+	if d.info.SpecFileType == datamodel.YAMLFileType {
+		return yaml.Marshal(d.info.RootNode)
+	} else {
+		yamlData, _ := yaml.Marshal(d.info.RootNode)
+		return utils.ConvertYAMLtoJSON(yamlData)
+	}
+}
+
 func (d *Document) BuildV2Document() (*DocumentModel[v2high.Swagger], []error) {
 	var errors []error
 	if d.info == nil {