cacheManager stalePeriod

## 🐛 Bug Report
my code
return ConstrainedBox(
constraints: BoxConstraints.expand(),
child: CachedNetworkImage(
cacheManager: CacheManager(Config(cacheKey,stalePeriod: const Duration(hours: 1),maxNrOfCacheObjects: 5)),
imageUrl: AppConfig.splash_url,
placeholder: (context,url)=>Image.asset('assets/ad/ad-default.jpg'),
errorWidget: (context, url, error)=>Image.asset('assets/ad/ad-default.jpg'),
fit: BoxFit.cover,),
);

### Expected behavior
const Duration(hours: 1) not work，i see file_service.dart has the following code,
@override
DateTime get validTill {
// Without a cache-control header we keep the file for a week
var ageDuration = const Duration(days: 7);
final controlHeader = _header(HttpHeaders.cacheControlHeader);
if (controlHeader != null) {
final controlSettings = controlHeader.split(',');
for (final setting in controlSettings) {
final sanitizedSetting = setting.trim().toLowerCase();
if (sanitizedSetting == 'no-cache') {
ageDuration = const Duration();
}
if (sanitizedSetting.startsWith('max-age=')) {
var validSeconds = int.tryParse(sanitizedSetting.split('=')[1]) ?? 0;
if (validSeconds > 0) {
ageDuration = Duration(seconds: validSeconds);
}
}
}
}

### Configuration
so i setting oss image,It is inconvenient

**Version:**3.0.0

**Platform:**
- [ ] :iphone: iOS
- [ ] :robot: Android