Airtime currency can be Enum

PR Summary

Enhancement in the to and add methods in Airtime The to and add methods in the Airtime function have been updated to now accept a currencyCode argument. This allows it to consider the currency type using either a string or a Currency object.
Removal of currencyIsInvalid method The currencyIsInvalid method was deemed unnecessary and thus removed from the codebase to simplify the process.
Updating the lessThanMinimumAmount method To offer more flexibility, the lessThanMinimumAmount method now accepts a traditional string as the currency code instead of rigidly requiring a Currency object.
Modification in AirtimeTransaction property initialization The initialization of the phoneNumber property in AirtimeTransaction has been revised to utilise PhoneNumber::make() instead of a direct string. This promotes more effective and correct phone number handling.
Incorporation of new test case in AirtimeTest A new test case has been added in tests/AirtimeTest.php to examine the functionality of adding a recipient using a Currency enum object. This ensures the robustness of the updated code.
Conservation of existing multiple recipient tests The previous test case for adding multiple recipients will remain intact and untouched, emphasizing that these new changes will have no effect on the already established functionalities.