Cache eviction

Hi, thank you for a great project simplifying persistent caching in flutter!

## 🚀 Feature Requests
Eviction based on cache size. It seems the only policy available is number of objects and age.

### Contextualize the feature
I only store immutable objects. No object can ever change. It therefore does not make sense to me to evict objects based on age. I wish that the user can set a maximum allowed storage capacity for the cache, such as 250MB and then fill this up, never evicting until the cache is full. Never evicting could also be important to make offline mode work smoother, though I realize offline support might be outside the scope of this project.

### Describe the feature

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

首先定位现有的、针对对象数量和对象年龄的缓存驱逐逻辑，然后检查缓存配置是如何对外提供的。确定可配置的最大存储容量应如何与当前策略交互，并在认为功能完成之前，通过测试覆盖 iOS 和 Android 上的行为。

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