Extend HTTP plugin by WithJsonBody(data)

AC:

- it should set header **"Accept": "application/json"**
- it should serialize to JSON using **JsonSerializer.SerializeToUtf8Bytes**

```csharp
var user = new UserData { Id = "1", Name = "anton" };

var request = Http.CreateRequest("GET", "https://nbomber.com")
                  .WithJsonBody(user);

var response = await Http.Send(httpClient, request);
```
