Move JsonExtensions and related tests to `core` module.
- Moved `decodeFromStringOrNull` and related testing from `app` and `authenticator` modules to `core` module.
- Updated all usages of `decodeFromStringOrNull` in `app` and `authenticator` to the new location in `core`.
- Deleted unused `JsonExtensionsTest` and `JsonExtensions.kt` in `app` and `authenticator`.
- Updated dependencies for core.
