Set the JsonReader as the default reader and use it in the YamlReader to reuse the ReadAsync() methods

Author: MaggieKimani1

src/Microsoft.OpenApi/Reader/OpenApiReaderRegistry.cs

@@ -14,6 +14,11 @@ public static class OpenApiReaderRegistry
     {
         private static readonly Dictionary<string, IOpenApiReader> _readers = new(StringComparer.OrdinalIgnoreCase);
 
+        /// <summary>
+        /// Defines a default OpenAPI reader.
+        /// </summary>
+        public static readonly IOpenApiReader DefaultReader = new OpenApiJsonReader();
+
         /// <summary>
         /// Registers an IOpenApiReader for a given OpenAPI format.
         /// </summary>