Add Concept Exercise Metadata files: `the-realm-of-echoes`

[0xNeshi]

Towards #103

Scaffold https://github.com/exercism/cairo/blob/main/exercises/concept/the-realm-of-echoes

TODO:

Use other implemented concept exercise metadata files for inspiration, e.g. https://github.com/exercism/cairo/tree/main/exercises/concept/lucians-luscious-lasagna.

• [x] instructions.md already implemented
• [ ] implement introduction.md
• [ ] implement hints.md
• [ ] implement ./meta/design.md
• [x] implement the example solution in .meta/exemplar.cairo
• [x] exercise stub at src/lib.cairo already implemented
• [x] tests for the exercise at _tests/the_realm_ofechoes.cairo already implemented
• [ ] replace <your_gh_username> placeholder with your actual GH username in .meta/config.json
• [ ] add missing blurb to .meta/config.json