Add QuickSend feature

[wneessen]

This PR introduces the QuickSend function for sending emails quickly with TLS and optional SMTP authentication. Added comprehensive unit tests to ensure QuickSend works correctly with different authentication mechanisms and handles various error scenarios.

This feature is similar to the SendMail method of the net/smtp package, but make use of all go-mail Client and Msg features, including error checking and validations.

This method will create a new client that connects to the server at addr, switches to TLS if possible, authenticates with the optional AuthData provided in auth and create a new simple Msg with the provided subject string and message bytes as body. The message will be sent using from as sender address and will be delivered to every address in rcpts. QuickSend will always send as text/plain ContentType.

For the SMTP authentication, if auth is not nil and AuthData.Auth is set to true, it will try to autodiscover the best SMTP authentication mechanism supported by the server. If auth is set to true but autodiscover is not able to find a suitable authentication mechanism or if the authentication fails, the mail delivery will fail completely.

The idea came, when I was talking to a developer that noted that for "sending a quick mail" the whole Client and Msg initialization is too "complex", while with net/smtp you can send out a mail much faster (even though you might run into a lot of things that can go wrong).

This feature now allows to send a mail with a single function call like this:

err := mail.QuickSend("server.domain.tld:587", mail.NewAuthData("username", "password"), "[email protected]", []string{"[email protected]"}, "This is a test message", []byte("the mail body goes into here.\r\nThanks for your attention."))
if err != nil {
	fmt.Printf("failed to send mail: %s\n", err)
	os.Exit(1)
}

[codecov]

Codecov Report

Attention: Patch coverage is 93.61702% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.64%. Comparing base (25a0fb2) to head (f05654d).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
quicksend.go	93.61%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
- Coverage   96.68%   96.64%   -0.05%     
==========================================
  Files          27       28       +1     
  Lines        3051     3098      +47     
==========================================
+ Hits         2950     2994      +44     
- Misses         70       72       +2     
- Partials       31       32       +1     

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