Completed adding v3 models (I think)

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

datamodel/low/3.0/security_scheme.go

@@ -0,0 +1,24 @@
+package v3
+
+import (
+    "github.com/pb33f/libopenapi/datamodel/low"
+    "gopkg.in/yaml.v3"
+)
+
+type SecurityScheme struct {
+    Node             *yaml.Node
+    Type             low.NodeReference[string]
+    Description      low.NodeReference[string]
+    Name             low.NodeReference[string]
+    In               low.NodeReference[string]
+    Scheme           low.NodeReference[string]
+    BearerFormat     low.NodeReference[string]
+    Flows            OAuthFlows
+    OpenIdConnectURL low.NodeReference[string]
+    Extensions       map[string]low.ObjectReference
+}
+
+type SecurityRequirement struct {
+    Node  *yaml.Node
+    Value []low.NodeReference[string]
+}