Add CreateXmlReader overload - adds Type parameter (#13302)

* Add CreateXmlReader overload - adds Type parameter

This change allows inheritors of XmlSerializerInputFormatter to return a different XmlReader depending on the type being serialized. For example, an inheritor could return XSD-validating readers that validate against one schema or another (or perhaps not validate at all), depending on the value of the type parameter.

src/Mvc/Mvc.Formatters.Xml/ref/Microsoft.AspNetCore.Mvc.Formatters.Xml.netcoreapp3.0.cs

@@ -47,6 +47,7 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
         protected override bool CanReadType(System.Type type) { throw null; }
         protected virtual System.Xml.Serialization.XmlSerializer CreateSerializer(System.Type type) { throw null; }
         protected virtual System.Xml.XmlReader CreateXmlReader(System.IO.Stream readStream, System.Text.Encoding encoding) { throw null; }
+        protected virtual System.Xml.XmlReader CreateXmlReader(System.IO.Stream readStream, System.Text.Encoding encoding, System.Type type) { throw null; }
         protected virtual System.Xml.Serialization.XmlSerializer GetCachedSerializer(System.Type type) { throw null; }
         protected virtual System.Type GetSerializableType(System.Type declaredType) { throw null; }
         [System.Diagnostics.DebuggerStepThroughAttribute]

src/Mvc/Mvc.Formatters.Xml/src/XmlSerializerInputFormatter.cs

@@ -119,8 +119,8 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
 
             try
             {
-                using var xmlReader = CreateXmlReader(readStream, encoding);
+                var type = GetSerializableType(context.ModelType);                
-                var type = GetSerializableType(context.ModelType);
+                using var xmlReader = CreateXmlReader(readStream, encoding, type);
 
                 var serializer = GetCachedSerializer(type);
 
@@ -191,6 +191,18 @@ namespace Microsoft.AspNetCore.Mvc.Formatters
             return wrapperProvider?.WrappingType ?? declaredType;
         }
 
+        /// <summary>
+        /// Called during deserialization to get the <see cref="XmlReader"/>.
+        /// </summary>
+        /// <param name="readStream">The <see cref="Stream"/> from which to read.</param>
+        /// <param name="encoding">The <see cref="Encoding"/> used to read the stream.</param>
+        /// <param name="type">The <see cref="Type"/> that is to be deserialized.</param>
+        /// <returns>The <see cref="XmlReader"/> used during deserialization.</returns>
+        protected virtual XmlReader CreateXmlReader(Stream readStream, Encoding encoding, Type type)
+        {
+            return CreateXmlReader(readStream, encoding);
+        }
+
         /// <summary>
         /// Called during deserialization to get the <see cref="XmlReader"/>.
         /// </summary>