Add SM owned files to code owners

[Thomas-Avery]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [X] Other

Objective

Add Secrets Manager owned files to code owners file.

Code changes

• .github/CODEOWNERS: Add Secrets Manager owned files to code owners file.

Before you submit

• Please add unit tests where it makes sense to do so