feat: auth-device-current prop

[dtodt]

Issue #3949 :

Description of changes:

• auth device current property added;
• cognito device current property added;
• fetchDevices method marks the current device (when exists);

The fetchDevices changes allow to conditionally forget remote devices, ex:

final devices = await _cognito().fetchDevices();
for (final device in devices) {
  if (device.current ?? false) continue;
  await _cognito().forgetDevice(device);
}

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