feat: Add log-level to background task message

[fregataa]

Let's add log_level field to background task event.
Current implementation of bgtask updates its progress with number type current & total progress and string type message.
Such implementation cannot inform clients of a granular failure of the total bgtask through progress reporter.
e.g. A task deleting multiple vfolders has failed to delete a few of them.

Adding log_level field could resolve this issue.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version

[fregataa]

• feat: Add log-level to background task message #2738 : 2 dependent PRs (#2737 , #2739 ) 👈 (View in Graphite)
• main

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[HyeockJinKim]

It seems better to manage this by creating code for the task results.
Additionally, the warning log should be logged where the failure occurs, not in the reporter.
We need to think of a way to pass the bg_task id values.

Please create an epic issue for both tasks. @fregataa