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