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.
Resolves#101.
Since we now have full control over the SMTP client we can also access the message input and output.
This PR introduces a new debug logging feature. Via the `Client.WithDebugLog` the user can enable this feature. It will then make use of the new `smtp/Client.SetDebugLog` method. Once the flag is set to true, the SMTP client will start logging incoming and outgoing messages to os.Stderr.
Log directions will be output accordingly
- Also import the original BSD-3-Clause.txt license from the Go team into the LICENSES directory
- Further on, license headers should hold "The go-mail Authors" instead of my name. Did this already for the MIT license.
As part of #97 we are going to fork the official `net/smtp` package into go-mail to provide us with more flexibility.
This commit fulfills the first big step of importing the package into smtp/. Also go-mail's own LoginAuth has been moved from auth/ into smtp/ to be consistent with the stdlib.
There are still a couple of open issues (i. e. license adjustments and making golangci-lint happy) but so far all tests already work, which is a good start.