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 11033773952

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 58 of 60 (96.67%) changed or added relevant lines in 2 files are covered.
• 1179 unchanged lines in 19 files lost coverage.
• Overall coverage decreased (-0.03%) to 64.679%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist-lua-ffi.cc	16	18	88.89%

Files with Coverage Reduction	New Missed Lines	%
pdns/dnsdistdist/dnsdist-crypto.cc	1	76.01%
pdns/misc.cc	1	63.26%
pdns/version.cc	3	95.57%
pdns/libssl.cc	3	59.01%
pdns/remote_logger.cc	3	53.36%
modules/gpgsqlbackend/spgsql.cc	3	67.7%
pdns/signingpipe.cc	5	83.11%
pdns/dnsdistdist/dnsdist-carbon.cc	6	62.16%
pdns/recursordist/syncres.cc	7	79.37%
pdns/packethandler.cc	8	72.69%

Totals
Change from base Build 10882883877:	-0.03%
Covered Lines:	124835
Relevant Lines:	162282

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💛 - Coveralls