Add Newtonsoft serializer to test project

add tests later, maybe

Test/Flurl.Test/Flurl.Test.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>net6.0;net48;net461</TargetFrameworks>
+    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
@@ -9,6 +10,7 @@
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.5.0" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="NUnit" Version="3.9.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.9.0" />
   </ItemGroup>

Test/Flurl.Test/Http/NewtonsoftJsonSerializer .cs

@@ -0,0 +1,45 @@
+using System.IO;
+using Newtonsoft.Json;
+
+namespace Flurl.Http.Configuration
+{
+	/// <summary>
+	/// ISerializer implementation based on Newtonsoft.Json.
+	/// Default serializer used in calls to GetJsonAsync, PostJsonAsync, etc.
+	/// </summary>
+	public class NewtonsoftJsonSerializer : ISerializer
+	{
+		private readonly JsonSerializerSettings _settings;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="NewtonsoftJsonSerializer"/> class.
+		/// </summary>
+		/// <param name="settings">Settings to control (de)serialization behavior.</param>
+		public NewtonsoftJsonSerializer(JsonSerializerSettings settings = null) {
+			_settings = settings;
+		}
+
+		/// <summary>
+		/// Serializes the specified object to a JSON string.
+		/// </summary>
+		/// <param name="obj">The object to serialize.</param>
+		public string Serialize(object obj) => JsonConvert.SerializeObject(obj, _settings);
+
+		/// <summary>
+		/// Deserializes the specified JSON string to an object of type T.
+		/// </summary>
+		/// <param name="s">The JSON string to deserialize.</param>
+		public T Deserialize<T>(string s) => JsonConvert.DeserializeObject<T>(s, _settings);
+
+		/// <summary>
+		/// Deserializes the specified stream to an object of type T.
+		/// </summary>
+		/// <param name="stream">The stream to deserialize.</param>
+		public T Deserialize<T>(Stream stream) {
+			// https://www.newtonsoft.com/json/help/html/Performance.htm#MemoryUsage
+			using var sr = new StreamReader(stream);
+			using var jr = new JsonTextReader(sr);
+			return JsonSerializer.CreateDefault(_settings).Deserialize<T>(jr);
+		}
+	}
+}

src/Flurl.Http/Configuration/DefaultJsonSerializer.cs

@@ -1,5 +1,4 @@
 using System.IO;
-using System.Security.Cryptography;
 using System.Text.Json;
 
 namespace Flurl.Http.Configuration