Expose HTTP.sys kernel response buffering control for HttpListener on Windows

[karimsalem1]

This PR adds an opt-in AppContext switch to enable kernel response buffering in HttpListener (Windows implementation only). When enabled, the flag HTTP_SEND_RESPONSE_FLAG_BUFFER_DATA is set on all calls to HttpSendHttpResponse and HttpSendResponseEntityBody (consistent with Win32 docs). This mirrors the behavior for setting the HttpSysOptions.EnableKernelResponseBuffering flag in ASP.NET HTTP.sys code.

Motivation in issue #123425.

Sample usage:

AppContext.SetSwitch("System.Net.HttpListener.EnableKernelResponseBuffering", true);

using var listener = new HttpListener();
listener.Prefixes.Add("http://localhost:8080/");
listener.Start();

Console.WriteLine("Listening...");
while (true)
{
    var context = await listener.GetContextAsync().ConfigureAwait(false);
    _ = Task.Run(() => HandleRequestAsync(context));
}

Notes:

• No public API changes
• Default behavior remains unchanged
• Since API is internal, tests use reflection to validate flag behavior.
• Added dedicated Windows-only test classes for HttpListener and HttpResponseStream, and consolidated existing Windows-only tests there for consistency.

TODO: add manual validation repro showing perf improvements using this feature

Thanks @ManickaP for all the help!

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[jkotas]

No public API changes

AppContext switches are de-facto public APIs, just a different kind.