[2.0.0] Rename Config to CacheManagerConfig

## 🏗 Enhancement Proposal

Since version 2.0.0 the flutter_cache_manager exports a class called `Config`. This name is very general and causes problems in all my projects because I have a general Config class.

I think the name `Config` is a very bad choice, as it is way to general. So I propose renaming it to `CacheManagerConfig` to prevent conflicts like these to occur.

I know that it is possible to work around this issue by using `import a as b`, but I think this should be avoided to make the development process more easy.

Also the name `CacheManagerConfig` would be much more intuitive to use, as you usually wouldn't expect to have a class called `Config`, but rather somthing like `CacheManager***` while writing your code with intellisense.

Start by locating the exported Config declaration and every reference to it in flutter_cache_manager. Review the package's existing tests and public API entry points before deciding the rename scope; done means the requested CacheManagerConfig name is used consistently and the affected tests pass.

索引モデルが issue の本文から書いたものです。