Fork the net/smtp package from Go's stdlib into go-mail

[wneessen]

As part of #97 we are going to fork the official net/smtp package into go-mail to provide us with more flexibility.

This PR forks net/smtp based on original Go stdlib commit: https://github.com/golang/go/commit/b2faff18ce28edad98303d2c3134dec1331fd7b5 into smtp/. Also go-mail's own LoginAuth has been moved from auth/ into smtp/ to be consistent with the stdlib.

The original license has been imported and the license headers have been adjusted to reflect the go-mail's MIT license and the original Go BSD-3-Clause license. Additionally, the license headers have been converted to SPDX style.

Finally the code and tests of the original code have been adjusted in a way that the complaints of golanci-lint were fixed.

[codecov-commenter]

Codecov Report

Merging #100 (2950f22) into main (a7126b4) will decrease coverage by 2.37%. The diff coverage is 69.72%.

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   83.26%   80.89%   -2.38%     
==========================================
  Files          15       20       +5     
  Lines        1548     1837     +289     
==========================================
+ Hits         1289     1486     +197     
- Misses        185      251      +66     
- Partials       74      100      +26     

Impacted Files	Coverage Δ
smtp/smtp.go	64.55% <64.55%> (ø)
smtp/auth_plain.go	81.81% <81.81%> (ø)
smtp/auth_cram_md5.go	91.66% <91.66%> (ø)
client.go	78.04% <100.00%> (-0.90%)	:arrow_down:
smtp/auth.go	100.00% <100.00%> (ø)
smtp/auth_login.go	96.00% <100.00%> (ø)
smtp/smtp_ehlo.go	100.00% <100.00%> (ø)

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