CountingSort[cpp]

#### This issue is about [CountingSort cpp].

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Start by browsing the repository’s existing C++ algorithm and data-structure implementations to identify the expected location and conventions for CountingSort. Add the requested CountingSort implementation in the established style, and verify that it matches the project’s existing organization and usage patterns.

Written by the indexing model from the issue text.