Add @CacheKey() annotation

[Axot017]

Add CacheKey annotation which will allow developers to pass function which will generate cache key (by default hashCode is used as a cache key).

• Create @CacheKey annotation that will take one param with type String Function(dynamic)
• Validate if @CacheKey is not used with @Ignore or @ignoreCache annotations
• Create generation logic that will replace hashKey with result of function passed in @CachedKey annotation
• Write generation tests
• Write integration tests
• Write dart doc for @cacheKey
• Add section in README.md for @cacheKey

Example usage:

String getListCacheKeys(Iterable<Object> l) {
  return l.map((e) => e.hashCode).join('')
}

@withCache
class Example {
  String doSomething(@CacheKey(getListCacheKeys) List<SomeObject> l) {
    return 'something';
  }
}

To discus:
Maybe we should create also something like @iterableCacheKey which will handle case described in "example usage" as its quite generic.

[RSC-88]

Hello, I think it is very good idea to create new decorator like "iterableCacheKey".

[dannnoon]

I believe it's a key feature 😏
We should think about what is the best way to generate hashcode for the list of items
Java way is to iterate through elements and multiply the list by the prime number, then add hash of the next item
result = result * prime + nextItem.hashCode