Use JsonContent when creating request bodies

[AustinWise]

This PR applies to the generated code for clients when using System.Text.Json serialization library.

This PR uses the JsonContent class to send objects as the request body. This is the documented way to serialize objects when making HTTP requests. It can be more efficient than serializing the entire request body into a byte array, particularly if the request body is large. This is because the JsonContent can stream the body while serializing and reuse buffers between requests.

In terms of the backwards compatibility impact of this change, the documentation says JsonContent was added in .NET 5. When running on .NET Framework, a reference to the System.Net.Http.Json Nuget package would now be required.

Contributes to #4083

[AustinWise]

I tested this PR with OpenAPI specification for Ollama I'm using. I confirmed that:

• Code generation is unchanged when using NewtonSoft.
• The code compiles and sends a request successfully with the new System.Text.Json code generation.

Here is what a diff of the generated code looks like:

                 using (var request_ = new System.Net.Http.HttpRequestMessage())
                 {
-                    var json_ = System.Text.Json.JsonSerializer.SerializeToUtf8Bytes(body, JsonSerializerSettings);
-                    var content_ = new System.Net.Http.ByteArrayContent(json_);
-                    content_.Headers.ContentType = System.Net.Http.Headers.MediaTypeHeaderValue.Parse("application/json");
+                    var content_ = System.Net.Http.Json.JsonContent.Create(body, System.Net.Http.Headers.MediaTypeHeaderValue.Parse("application/json"), JsonSerializerSettings);
                     request_.Content = content_;