Implement SMTP client debug logging

[wneessen]

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 logged accordingly.

• C --> S means Client to Server communication
• C <-- S means Server to Client communication

Example:

[codecov-commenter]

Codecov Report

Merging #102 (ccbab59) into main (813020f) will decrease coverage by 0.27%.
The diff coverage is 64.51%.

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
- Coverage   81.05%   80.78%   -0.28%     
==========================================
  Files          20       20              
  Lines        1837     1868      +31     
==========================================
+ Hits         1489     1509      +20     
- Misses        249      258       +9     
- Partials       99      101       +2     

Impacted Files	Coverage Δ
smtp/smtp.go	63.92% <55.55%> (-0.64%)	⬇️
client.go	78.85% <76.92%> (-0.08%)	⬇️

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[lightglitch]

Instead of using the golang logger with not use an interface for the logger like for example:

type Logger interface {
	Errorf(format string, v ...interface{})
	Warnf(format string, v ...interface{})
	Debugf(format string, v ...interface{})
}

with mail.WithDebugLog(log) so that we can plug any logger that we want into the library?

[wneessen]

Good point. I'll check what would be the best option here.