Seperate Audit Log file

[blizzz]

Make it possible to write to a seperate audit log file, instead of the nextcloud log

Rough overview of tasks:

• put reusable parts of \OC\Log\File to the public namespace (e.g. as trait or abstract class)
• extend the rotate logic to accept other log files (or also move parts to public space)
• implement an ILogger for admin_audit (perhaps having parts of \OC\Log in public space makes sense, to evaluate).
• add appconfig options for admin_audit to override the log location (and type), defaulting to the main log file location.

[blizzz]

Limitation: contents of the custom audit log file won't be visible in Admin → Logging (same behaviour as if using Syslog, e.g.).

[MorrisJobke]

Fixed in #9293