feat: ability to disable sync job by configuration

[madbob]

Here I've added the ability to disable the syncronization job by settings -1 as value of app.mail.background-sync-interval.
I've avoided to use 0 as special value to avoid breaking existing behavior (which is: when 0, fallback to the default interval).

I've not found better ways to disable the job "on demand". References I've found in other Nextcloud repositories (in files_antivirus and bookmarks) adopt all the same approach: just return from the run() method if the condition of disabling is true.

Fixes #10287

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[ChristophWurst]

LGTM! Thanks