Merge pull request #6411 from rgacogne/dnsdist-dynamic-healthchecks

dnsdist: Add `checkFunction` to implement a dynamic health check

diff --cc pdns/dnsdist-lua.cc
Simple merge

diff --cc pdns/dnsdist.cc
Simple merge

diff --cc pdns/dnsdist.hh
Simple merge

diff --cc pdns/dnsdistdist/docs/reference/config.rst
Simple merge

diff --cc regression-tests.dnsdist/dnsdisttests.py
Simple merge