Introduce Android support

[kevinchalet]

This PR introduces built-in support for Android API 21+ (Android 5.0 and higher).

Note: unlike the iOS integration, Android requires defining a custom Activity to handle callbacks. E.g on MAUI:

[Activity(NoHistory = true, LaunchMode = LaunchMode.SingleTop, Exported = true)]
[IntentFilter([Intent.ActionView],
    Categories = [Intent.CategoryDefault, Intent.CategoryBrowsable],
    DataScheme = "com.openiddict.sandbox.maui.client")]
public class CallbackActivity : Activity
{
    protected override async void OnCreate(Bundle? savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        if (Intent is not Intent intent)
        {
            return;
        }

        var provider = IPlatformApplication.Current?.Services ??
            throw new InvalidOperationException("The DI provider cannot be resolved.");
        var service = provider.GetRequiredService<OpenIddictClientSystemIntegrationService>();

        try
        {
            await service.HandleCustomTabsIntentAsync(intent);
        }

        finally
        {
            Finish();
        }
    }
}