Challenge: Build a Custom Collector for Category Counting

You need to implement a custom Collector that processes a list of products and counts how many products belong to each category.

1. In the supplier() method, return a new HashMap.
2. In the accumulator() method, call the get() method on the product map to retrieve the value of the "category" key and pass it as the first parameter of the merge() method.
3. For the second parameter in the same method, provide the initial value (1).
4. In the combiner() method, pass the key of map2 as the first parameter of the merge() method.
5. For the second parameter in the same method, pass the value of the map2.
6. In the finisher() method, return the map without modifying it using a lambda.
7. In the characteristics() method, return a set that ensures the collection remains unchanged (IDENTITY_FINISH).
8. In the collect() method, pass the implementation of the Collector interface (CategoryCountCollector).