A `const CallDepth` placeholder has been introduced and has been set to 2. This is used as an argument in Output method calls in various log levels like debug, info, warn, and error in place of previously hardcoded '2'. This change improves the readability of the code. The '2' represented the call depth of log.Logger's Output method, which wasn't immediately clear with the hardcoded value.
As stated in https://github.com/wneessen/go-mail/pull/102#issuecomment-1411956040 it would be beneficial if, instead of forcing the Go stdlib logger on the user to provide a simple interface and use that for logging purposes.
This PR implements this simple log.Logger interface as well as a standard logger that satisfies this interface. If no custom logger is provided, the Stdlog will be used (which makes use of the Go stdlib again).
Accordingly, a `Client.WithLogger` and `Client.SetLogger` have been implemented. Same applies for the smtp counterparts.
