auth: allow EDNS Client Subnet prefix to be narrowed

[miodvallat]

Short description

This PR adds two new settings, edns-scope-mask-ipv4 and edns-scope-mask-ipv6, which will be used to narrow the scope mask in EDNS responses, except for packets which have hit a view, in which case the view netmask is still used.

Note that this PR is built ontop the first commit of #17878. No need to review more than once.

This could help people mitigate #16803.

Todo:

• tests
• documentation

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Coverage Report for CI Build 32354577197

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage decreased (-0.02%) to 71.205%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 11 of 11 lines across 2 files are fully covered (100%).
• 61 coverage regressions across 15 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

61 previously-covered lines in 15 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
pdns/misc.cc	11	62.54%
pdns/dnsproxy.cc	6	55.61%
pdns/recursordist/lwres.cc	6	66.78%
pdns/recursordist/syncres.cc	6	81.51%
pdns/recursordist/test-syncres_cc1.cc	6	80.18%
pdns/dnsdistdist/dnsdist-carbon.cc	4	59.8%
pdns/dnspacket.cc	4	72.71%
pdns/shuffle.cc	4	53.93%
pdns/recursordist/test-syncres_cc2.cc	3	79.39%
pdns/stubresolver.cc	3	75.0%

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Coverage Stats

Relevant Lines:	173289
Covered Lines:	135297
Line Coverage:	78.08%
Relevant Branches:	82898
Covered Branches:	47120
Branch Coverage:	56.84%
Branches in Coverage %:	Yes
Coverage Strength:	5871217.09 hits per line

---

💛 - Coveralls

[Habbie]

first commit (from other PR) already looked good to me. Second commit (wrapup cleanup) looks good to me but deserves another pair of eyes.

Third commit also looks good, but needs docs. I'd also like rec and dnsdist people to have a brief look (perhaps once docs are written) to make sure all our products interact correctly with this change.