[Feature] Custom default current path

[gavinliu]

The current version calls up the picker to locate all photos by default. It is expected to add a function that can customize a certain address to open by default. For example, the screenshot album is opened by default.

当前版本唤起选择器默认定位的是全部照片，期望增加一个可以自定义默认打开某个地址的功能，比如默认打开截图相册。

[gavinliu]

我预期在 asset_picker_provider 的 getPaths 函数中第一次为 _currentPath 赋值时，添加自定义的功能，如下位置：

flutter_wechat_assets_picker/lib/src/provider/asset_picker_provider.dart

Lines 388 to 390 in 7f06b77

	if (_paths.isNotEmpty) {
	_currentPath ??= _paths.first;
	}

但实际运行效果和预期不太相符。

---

原因是 asset_picker_provider 的加载逻辑是，先加载全部，再刷新所有目录。 不太明白为什么要这样设计 ？为了性能？

flutter_wechat_assets_picker/lib/src/provider/asset_picker_provider.dart

Lines 277 to 280 in 7f06b77

	Future<void>.delayed(initializeDelayDuration, () async {
	await getPaths(onlyAll: true);
	await getPaths(onlyAll: false);
	});