dnsdist: Add a FFI accessor to incoming proxy protocol values

[rgacogne]

Short description

Needs:

• manual testing
• unit tests
• regression tests

Fixes https://github.com//issues/14656

Checklist

I have:

• read the CONTRIBUTING.md document
• 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]

Pull Request Test Coverage Report for Build 10899745169

Details

• 59 of 59 (100.0%) changed or added relevant lines in 2 files are covered.
• 40 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.02%) to 64.728%

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Files with Coverage Reduction	New Missed Lines	%
ext/json11/json11.cpp	1	64.49%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/recursordist/syncres.cc	1	79.54%
pdns/misc.cc	1	63.09%
modules/lmdbbackend/lmdbbackend.cc	2	73.54%
pdns/recursordist/recpacketcache.hh	3	89.55%
modules/gpgsqlbackend/spgsql.cc	3	67.7%
pdns/recursordist/test-syncres_cc1.cc	6	89.59%
pdns/dnsdistdist/dnsdist-carbon.cc	6	61.89%
pdns/packethandler.cc	8	72.65%

Totals
Change from base Build 10882883877:	0.02%
Covered Lines:	124959
Relevant Lines:	162342

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💛 - Coveralls