Updated Walker to use separate methods for each element

Author: darrelmiller

src/Microsoft.OpenApi/Services/OpenApiVisitorBase.cs

@@ -146,6 +146,13 @@ public virtual void Visit(OpenApiMediaType mediaType)
         {
         }
 
+        /// <summary>
+        /// Visits <see cref="OpenApiEncoding"/>
+        /// </summary>
+        public virtual void Visit(OpenApiEncoding encoding)
+        {
+        }
+
         /// <summary>
         /// Visits the examples.
         /// </summary>