feat: add smtp auth login support

[buroa]

This let's providers like smtp.office365.com work.

[buroa]

Extremely easy PR for you @foxcpp :)

[foxcpp]

Needs corresponding documentation update

[buroa]

@foxcpp Done!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 52.38%. Comparing base (1d6cd8c) to head (a87a9c0).

Files	Patch %	Lines
internal/target/smtp/sasl.go	0.00%	7 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #683   +/-   ##
=======================================
  Coverage   52.38%   52.38%           
=======================================
  Files         128      128           
  Lines       13382    13389    +7     
=======================================
+ Hits         7010     7014    +4     
- Misses       5707     5711    +4     
+ Partials      665      664    -1     

Flag	Coverage Δ
integration	34.11% <0.00%> (-0.12%)	:arrow_down:
unit	53.76% <0.00%> (+<0.01%)	:arrow_up:

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[buroa]

Let me add some tests as well @foxcpp.

[buroa]

@foxcpp Ready for you.

[buroa]

@foxcpp Can we get this merged?

[buroa]

@foxcpp Ping