Support log rotation customization in fluent.conf

[kenhys]

Is your feature request related to a problem? Please describe.

In the current version, log rotation options were supported as
--log-rotate-age or --log-rotate-size via command line options.

On Windows, if you use td-agent package, fluentd is launched as a
windows service, it is required to configure again via
--reg-winsvc-fluentdopt or edit fluentdopt registry key for
customization.

This approach is not convenient for Windows (especially td-agent)
users, so it may be better to support more comprehensive solution -
customize via configuration file.

Describe the solution you'd like

Add configuration parameter to customize by log section.

<system>
  <log>
    rotate_age 5
    rotate_size 1048576
  </log>
</system>

Describe alternatives you've considered

N/A

Additional context

If this approach is implemented, we need to update https://docs.fluentd.org/deployment/system-config

working on #3352