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 31175548763

Coverage increased (+9.4%) to 71.205%

Details

• Coverage increased (+9.4%) from the base build.
• Patch coverage: 41 of 41 lines across 3 files are fully covered (100%).
• 23753 coverage regressions across 295 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

23753 previously-covered lines in 295 files lost coverage.

Top 10 Files by Coverage Loss	Lines Losing Coverage	Coverage
pdns/dnsdistdist/dnsdist-lua.cc	1032	52.04%
pdns/lua-record.cc	912	0.16%
pdns/recursordist/rec-main.cc	778	65.83%
pdns/recursordist/syncres.cc	611	81.52%
pdns/dnsdistdist/dnsdist.cc	607	68.8%
modules/lmdbbackend/lmdbbackend.cc	579	72.42%
pdns/recursordist/rec_channel_rec.cc	561	36.35%
pdns/dnsdistdist/dnsdist-configuration-yaml.cc	543	55.78%
pdns/dnsdistdist/dnsdist-actions-factory.cc	503	62.71%
pdns/recursordist/ws-recursor.cc	502	22.03%

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Coverage Stats

Relevant Lines:	173021
Covered Lines:	135094
Line Coverage:	78.08%
Relevant Branches:	82778
Covered Branches:	47047
Branch Coverage:	56.84%
Branches in Coverage %:	Yes
Coverage Strength:	6860912.5 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.