Support writing to a stream

[dbrattli]

So we don't have to generate the full string.

• https://www.stevejgordon.co.uk/using-the-bodyreader-and-bodywriter-in-asp-net-core-3-0
• https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware/request-response?view=aspnetcore-3.1
• https://devblogs.microsoft.com/dotnet/system-io-pipelines-high-performance-io-in-net/