Add column event_time to system table backup_log system

[jaymebrd]

Adds event_time to backup_log system table.

Other *_log system tables have column event_time.
backup_log only has event_time_microseconds which most other system tables don't have.

It is convenient for scripts/monitoring/dashboards to rely on having a consistent column to query across _log system tables.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

For consistency with other system tables, system.backup_log now has a column event_time.

[robot-clickhouse]

This is an automated comment for commit edb22a8 with description of existing statuses. It's updated for the latest CI running

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Check name	Description	Status
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Stateful tests	Runs stateful functional tests for ClickHouse binaries built in various configurations -- release, debug, with sanitizers, etc	❌ failure
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Upgrade check	Runs stress tests on server version from last release and then tries to upgrade it to the version from the PR. It checks if the new server can successfully startup without any errors, crashes or sanitizer asserts	❌ failure

Successful checks

Check name	Description	Status
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