Support external logger

[darkstarx]

Hi!
Thanks for the great package!

Can you please support an external logger as an optional alternative to the internal print? Something like:

import 'dart:developer';

SmtpClient.logger = log;

where log is

void log(String message, {
  DateTime? time,
  int? sequenceNumber,
  int level = 0,
  String name = '',
  Zone? zone,
  Object? error,
  StackTrace? stackTrace,
});
``` from the `dart:developer`.

and in your `ClientBase.log` method smth like:
```dart
typedef Logger = void Function(String message, {String name});

/// An optional external logger.
Logger? logger;

void log(final Object? logObject, {
  final bool isClient = true,
  String? initial,
})
{
  if (isLogEnabled) {
    initial ??= (isClient == true) ? initialClient : initialServer;
    if (logger != null) {
      logger(logObject?.toString() ?? '',
        name: initial,
      );
    } else if (logName != null) {
      logger()
      print('$logName $initial: $logObject');
    } else {
      print('$initial: $logObject');
    }
  }
}

So it will be compatible with dart:developer and package:logging at the choice of the package user.
As an alternative, you could use package:logging inside of your package and provide an access to the log level of this logger to set the log level externally (like in the hotreloader package).

Thanks again for your package, guys!