core: add option to anonymize external command user in icinga log #5689

Adds a new icinga.cfg option "log_anonymized_external_command_author".
Config description:
  LOG ANONYMIZED EXTERNAL COMMAND AUTHOR !!EXPERIMENTAL!!
  This option substitutes the user name on external commands with
  the string "<anon>" if the command gets logged. It is
  anonymized in log files only. This option was added to make
  icinga compliant with data retention laws on various countries.
  This option is disabled by default.

Refs: #5689
whatthecommit: Either Hot Shit or Total Bollocks
core: add changelogentry for #5689
core: anonymize external command - add tests and config change #5689

* Add file with config change description
* Satisfy tests

Refs: #5689
whatthecommit: avoid the cherries
Merge branch 'feature/anonymize-external-command-user-5689' into next

Fixes #5689