scripts: add DKIM helper script for Rspamd

[georglauterbach]

Description

DISCLAIMER: I am sure you think I must be joking looking at the diff size, but I am really not sure how to put this into multiple PRs, because all these changes additions really belong together (updating docs without adding the new script doesn't make sense; not adding docs but only the script would be incoherent; not adding tests would result in code being deployed that is not tested at all - you see the issue). If you think this PR can be split up into multiple PRs, please let me know.

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This PR is the last PR before we can go into feature-freeze for v12.1.0 Nevermind, we also need adjust F2B (revert some of the recent changes, see #3288). It adds support for DKIM signing keys via a helper script for Rspamd. The mode of operation is mostly identical to the already present open-dkim script.

As a good programmer, I added tests that show my changes additions are effective. I also updated the documentation for DKIM signing with Rspamd.

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@polarathene I have not assigned you as a reviewer because I think for the script additions (and this PR mostly is one big script addition including tests) @casperklein will do a good job when it comes to reviewing. When you want to have a look at the docs changes though (0ac887b), I would not mind :) The docs preview should help, because the docs-diffs are always a bit fuzzy.

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Closes #3249

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[polarathene]

I likely won't have time to participate in review. Just had a quick peek and noticed you're maintaining both opendkim and rspamd methods of generating the DKIM keys.

There was an agnostic method I shared earlier. However, since you plan to drop opendkim eventually, perhaps that agnostic approach is not as useful.

[georglauterbach]

I likely won't have time to participate in review.

Hence I didn't assign you in the first place :)

Just had a quick peek and noticed you're maintaining both opendkim and rspamd methods of generating the DKIM keys.

There was an agnostic method I shared earlier. However, since you plan to drop opendkim eventually, perhaps that agnostic approach is not as useful.

Exactly :)

[georglauterbach]

@casperklein this PR is rather big, hence I wanted to ask you whether you actually have time to review this. If you're very short on time, I got to admit that I feel rather confident about these changes, so you could also do a very light review. They may not be perfect, but few changes really are. Just tell me so I get the rough picture.

[casperklein]

I'll review this tonight.

[georglauterbach]

I'll review this tonight.

Very nice 🚀👍🏼

[casperklein]

Nothing big. Some change requests and some comments you might aggree with and change the code or leave it as it is..

[casperklein]

Looks good, but one test is failing.

[georglauterbach]

My tests seem to work. I changed the wording on an error message and the tests check for the exact wording.. easily corrected.

UPDATE: Done in 057e203.

[github-actions]

Documentation preview for this PR is ready! 🎉

Built with commit: be78d48