RedBlackTree [C++]

#### This issue is about [RedBlackTree](https://github.com/iiitv/algos).

- [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 reviewing the RedBlackTree context in the linked iiitv/algos repository and comparing existing data-structure implementations. No source file or test is named in the issue, so first identify the repository convention for C++ additions. Done means a complete RedBlackTree implementation integrated consistently with the project.

Written by the indexing model from the issue text.