[C++] Extract Concepts from v2 exercise: etl

Extract the list of concepts required to solve the v2 C++ exercise: `etl`. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches (you can find community solutions here: `https://exercism.io/tracks/cpp/exercises/etl/solutions`). The goal of this task is to help determine the total list of concepts that make up `etl`.

The list of concepts should be stored in the following document: `languages/cpp/reference/exercise-concepts/etl.md`. [Use this link to create a PR to add this document][pr-link].

[pr-link]: ../new/master?filename=languages/cpp/reference/exercise-concepts/etl.md