Adds deserialization support for JsonDocument

This change adds support to `System.Text.Json` for deserializing `JsonDocument`.

Specifically, an internal converter `JsonDocumentConverter` is added to the default converter dictionary.

I have created a basic test, but I feel more could be done here, and it may not be in the right file/class - some guidance would be helpful here.

Fixes dotnet#1573

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -101,6 +101,7 @@
     <Compile Include="System\Text\Json\Serialization\Converters\Value\Int32Converter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Value\Int64Converter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Value\JsonElementConverter.cs" />
+    <Compile Include="System\Text\Json\Serialization\Converters\Value\JsonDocumentConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Value\KeyValuePairConverter.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Value\KeyValuePairConverterFactory.cs" />
     <Compile Include="System\Text\Json\Serialization\Converters\Value\NullableConverter.cs" />

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/JsonDocumentConverter.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Text.Json.Serialization.Converters
+{
+    internal sealed class JsonDocumentConverter : JsonConverter<JsonDocument>
+    {
+        public override JsonDocument Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            return JsonDocument.ParseValue(ref reader);
+        }
+
+        public override void Write(Utf8JsonWriter writer, JsonDocument value, JsonSerializerOptions options)
+        {
+            value.WriteTo(writer);
+        }
+    }
+}

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.Converters.cs

@@ -37,7 +37,7 @@ public sealed partial class JsonSerializerOptions
 
         private static Dictionary<Type, JsonConverter> GetDefaultSimpleConverters()
         {
-            const int NumberOfSimpleConverters = 22;
+            const int NumberOfSimpleConverters = 23;
             var converters = new Dictionary<Type, JsonConverter>(NumberOfSimpleConverters);
 
             // Use a dictionary for simple converters.
@@ -55,6 +55,7 @@ private static Dictionary<Type, JsonConverter> GetDefaultSimpleConverters()
             Add(new Int32Converter());
             Add(new Int64Converter());
             Add(new JsonElementConverter());
+            Add(new JsonDocumentConverter());
             Add(new ObjectConverter());
             Add(new SByteConverter());
             Add(new SingleConverter());

src/libraries/System.Text.Json/tests/NewtonsoftTests/JsonSerializerTests.cs

@@ -131,6 +131,15 @@ public void PreserveReferencesCallbackTest()
             p2.Spouse = p1;
             Assert.Throws<JsonException> (() => JsonSerializer.Serialize(p1));
         }
+
+        [Fact]
+        public void JsonDocumentDeserialization()
+        {
+            using (JsonDocument document = JsonSerializer.Deserialize<JsonDocument>("{\"data\": 12}"))
+            {
+                Assert.Equal(12, document.RootElement.GetProperty("data").GetInt32());
+            }
+        }
     }
 
     public class PersonReference