dnsdist: Add setVerbose() to switch the verbose mode at runtime

[rgacogne]

Short description

And also from the configuration, without having to edit the arguments passed to the program.

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)

[rgacogne]

CI failure is unrelated:

   FAIL: testA (test_SimpleForwardOverDoT.testSimpleForwardOverDoT)
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  Traceback (most recent call last):
    File "/home/runner/work/pdns/pdns/regression-tests.recursor-dnssec/test_SimpleForwardOverDoT.py", line 36, in testA
      self.assertMessageIsAuthenticated(res)
    File "/home/runner/work/pdns/pdns/regression-tests.recursor-dnssec/recursortests.py", line 856, in assertMessageIsAuthenticated
      self.assertTrue('AD' in msgFlags, "No AD flag found in the message for %s" % msg.question[0].name)
  AssertionError: False is not true : No AD flag found in the message for dns.google.
  
  ----------------------------------------------------------------------