[Feature Request]: Allow an optional ID in CashTransaction

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/micropowermanager/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/micropowermanager/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/micropowermanager/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Currently there is no way to unique distinguish cash transactions. This might be necessary when transactions are also tracked and managed in other systems.

### Proposed Solution

When registering CashTransactions please allow users to provide an optional external transaction ID. System should throw an error when a duplicated transactions is registered.

### Alternatives Considered

We request to appear ID transactions for the payments.

### Additional Information

If present external_transaction_id should be included in any exports.