All methods below should be implemented in CollectionService class. To complete tasks use Stream API methods.

Implement the method public List<Integer> filterEvenNumbers(List<Integer> list). It must filter list and return only even numbers. To complete the task use filter(...) method in Stream API.

Implement the method public List<String> toUpperCaseCollection(List<String> list). It must modify a list and return all values in upper case. To complete the task use map(...) method in Stream API.

Implement the method public Optional<Integer> findMax(List<Integer> list). It must find max number in the list. To complete the task use max(...) method in Stream API.

Implement the method public Optional<Integer> findMin(List<List<Integer>> list). It must find min value among all lists. To complete the task use flatMap(...) and min(...) methods in Stream API.

Implement the method public Integer sum(List<Integer> list). It must calculate a sum of all elements in the list. To complete the task use reduce(...) method in Stream API.