dnsdist: Add the ability to set custom HTTP responses over DoH3

[rgacogne]

Short description

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)

[miodvallat]

Looking gud.

[coveralls]

Pull Request Test Coverage Report for Build 12711285724

Details

• 81 of 83 (97.59%) changed or added relevant lines in 4 files are covered.
• 57 unchanged lines in 12 files lost coverage.
• Overall coverage increased (+0.02%) to 64.941%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/dnsdistdist/dnsdist-lua-actions.cc	20	22	90.91%

Files with Coverage Reduction	New Missed Lines	%
pdns/recursordist/aggressive_nsec.cc	2	66.02%
pdns/dnsdistdist/dnsdist.cc	2	68.98%
pdns/misc.hh	3	87.62%
pdns/remote_logger.cc	3	53.81%
pdns/misc.cc	3	65.02%
pdns/iputils.cc	3	55.91%
modules/godbcbackend/sodbc.cc	4	70.8%
pdns/recursordist/syncres.cc	5	80.09%
pdns/rcpgenerator.cc	5	90.46%
pdns/dnsdistdist/dnsdist-carbon.cc	6	61.87%

Totals
Change from base Build 12710659725:	0.02%
Covered Lines:	126350
Relevant Lines:	163744

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💛 - Coveralls

[rgacogne]

Rebased to fix a conflict.