dnsdist-1.9.x: Backport 14664 - Add a FFI accessor to incoming proxy protocol values

[rgacogne]

Short description

Backport of #14664 to rel/dnsdist-1.9.x

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)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 11052292878

Details

• 58 of 60 (96.67%) changed or added relevant lines in 2 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 55.783%

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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/pollmplexer.cc	1	82.39%
pdns/dnsdist-tcp.cc	8	76.42%

Totals
Change from base Build 10994261869:	0.03%
Covered Lines:	48079
Relevant Lines:	65633

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💛 - Coveralls