Support of S/MIME Signing

[theexiile1305]

This pull request resolve #296.

[theexiile1305]

@wneessen Now the PR is ready to be reviewed. Many thanks in advance.

Should I also add a notice in the README, so that S/MIME singing is now supported?

[theexiile1305]

@wneessen The PR has now been improved again so that the README.md has been updated and an intermediate certificate can be added. These are also best practices for handling certificates.

I would like to take this opportunity to thank my contact at DFN-CERT for checking the handling of the certificates.

[wneessen]

As discussed we'll merge this as experimental with only "plain" mail support at this point. Multi-part support will follow later.

[codecov]

Codecov Report

Attention: Patch coverage is 61.37339% with 180 lines in your changes missing coverage. Please review.

Project coverage is 91.29%. Comparing base (8646857) to head (1821cc7).
Report is 54 commits behind head on main.

Files with missing lines	Patch %	Lines
pkcs7.go	52.99%	94 Missing and 16 partials ⚠️
msg.go	52.38%	29 Missing and 11 partials ⚠️
smime.go	75.00%	20 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
- Coverage   96.87%   91.29%   -5.59%     
==========================================
  Files          28       30       +2     
  Lines        3139     3595     +456     
==========================================
+ Hits         3041     3282     +241     
- Misses         68      251     +183     
- Partials       30       62      +32     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[wneessen]

Even though some changes and tests are still needed (as discussed), I will now merge this completely instead of cherry-picking and then commit the changes in a seperate PR. Thanks again @theexiile1305 for all the efforts you put into this PR!