[WIP] Create entity interfaces for Channel, Currency and Locale

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Almost
| Tests pass?          | 
| Scenarios pass?      |
| Checkstyle issues?*  |
| PMD issues?**        |
| Changelog updated?   |
| Fixed tickets        |
| DB schema updated?   |
| Migration script?    |
| Doc PR               |