HTTP local server fallback for WebAuthenticationBroker

[maxkatz6]

Closes #26

System OAuth WebAuthenticationBroker implementation is not always available. And NativeWebDialog implementation might not be desirable (as it's not connected with user browser sessions).

In the wild, many apps implement HTTP listener, including our own tooling. This new WebAuthenticationBroker brings the same fallback to this library.

New API:

public record WebAuthenticatorOptions(Uri RequestUri, Uri RedirectUri)
{
+   public WebAuthenticatorMode Mode { get; init; }

+   [Obsolete("Use Mode = WebAuthenticatorMode.NativeWebDialog instead.")]
    public bool PreferNativeWebDialog { get; init; }

    public bool NonPersistent { get; init; }

    public Func<NativeWebDialog?>? NativeWebDialogFactory { get; init; }

+   public BrowserOptions? BrowserOptions { get; init; }
}

public enum WebAuthenticatorMode
{
    Auto,

    [SupportedOSPlatform("macos")]
    [SupportedOSPlatform("ios")]
    [SupportedOSPlatform("android")]
    [SupportedOSPlatform("browser")]
    System,

    [SupportedOSPlatform("windows")]
    [SupportedOSPlatform("macos")]
    [SupportedOSPlatform("android")]
    [SupportedOSPlatform("linux")]
    NativeWebDialog,

    [UnsupportedOSPlatform("browser")]
    Browser
}

public record BrowserOptions
{
    public TimeSpan Timeout { get; init; } = TimeSpan.FromMinutes(5);

    public BrowserResponseHandler? ResponseHandler { get; init; }

    public BrowserCallbackFilter? CallbackFilter { get; init; }

    public delegate Task BrowserResponseHandler(
        WebAuthenticationResult result, BrowserResponse response);

    public delegate bool BrowserCallbackFilter(
        WebAuthenticationResult result);
}

public sealed class BrowserResponse
{
    public HttpStatusCode StatusCode { get; set; }
    public Stream OutputStream { get; }
    public void Redirect(Uri uri)
}