Allow ignoring specific files using a configuration property

[lars-sh]

Hi everyone,

this Pull Request does not follow any standards as I'm not involved in PHP or Nextcloud development usually.
Though, I was working on a quick-fix to implement some simple way to ignore false-positive findings and it might be interesting for others, too.

This might relate to #88 and #178.
Feel free to copy, rework or modify my changes.

The idea is to introduce a simple config.php property called antivirus_ignore that takes either an array of full file paths or a string that needs to be a valid regular expression which needs to match the file path using preg_match. If so, the file is ignored and no more warning is reported.

You can check this e.g. using occ files_antivirus:scan on the command line but it applies to the background scanner in the same way.

[github-actions]

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