Make treshold the 1 day treshold for maxObjects configurable

## 🏗 Enhancement Proposal

When setting maxNrOfCacheObjects to 100 the cache can still grow above that size. This is caused by logic in the CacheObjectProvider in the getObjectsOverCapacity function.
```
where: '${CacheObject.columnTouched} < ?',
whereArgs: [
DateTime.now().subtract(const Duration(days: 1)).millisecondsSinceEpoch
],
```
From an this issue #349 I understand the reasoning behind this default value.
### Pitch

For app which are not used very intense I think the default makes sense. However for apps which use a lot of images and are used intensively throughout the day this results in a very big local cache. I've got reports of several GB.

I would argue that the there should be a parameter to override this in the Config object. This would be in line with for example the maxObjects and stalePeriod parameters.

Concretely:
- Add parameter to Config ObjectMinimumRetention (open for better naming)
- Type Duration
- Optional, default value 1 day
- Pass and use this parameter to the CacheObjectProvider
- Document

I'm happy to do a PR on this as you think this makes sense.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android

从 CacheObjectProvider.getObjectsOverCapacity 开始，跟踪 Config 对象当前如何将 maxObjects 和 stalePeriod 传入 provider。添加可选的 Duration 设置，默认值为一天，通过 provider 传递该设置，与相关配置选项一并记录文档，并验证缓存是否遵守配置的保留阈值。

由索引模型根据 Issue 内容生成。