[login_client_flutter] Allow configuring `FlutterSecureStorage`

[PiotrRogulski]

The FlutterSecureCredentialsStorage class provided in login_client_flutter creates the underlying storage with the default options. This makes it impossible to configure it (for example, I'd like to set a different KeyChain accessibility on iOS). I suggest adding an optional storage parameter to FlutterSecureCredentialsStorage, enabling passing custom configurations.