Add an option to skip adding a User-Agent

[gegorov2030]

Adds an option to omit the default User-Agent, suitable for scenarios like SMTP proxies where headers are conditionally passed based on receipt.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 75.45%. Comparing base (68e6284) to head (299490f).

Files	Patch %	Lines
msg.go	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
- Coverage   81.29%   75.45%   -5.85%     
==========================================
  Files          24       24              
  Lines        2128     2135       +7     
==========================================
- Hits         1730     1611     -119     
- Misses        283      417     +134     
+ Partials      115      107       -8     

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[wneessen]

Thanks for the PR @gegorov2030. Can you please add a test for this as well? Besides that, LGTM.

[gegorov2030]

Added, thank you