Improve time window to use cron expression.

[gthao313]

Right now, we only support time window scheduler to allow brupop update on specific a time slot around a day.
For example:

bottlerocket-update-operator/.env

Lines 29 to 33 in eebfbcd

	# Update time window setting
	# Brupop will operate node updates within update time window.
	# when you set up time window start and stop time, you should use UTC (24-hour time notation).
	UPDATE_WINDOW_START=0:0:0
	UPDATE_WINDOW_STOP=0:0:0

Cron expression would be a better choice to enhance time window scheduler, so users are more flexible to schedule maintenance time window on their purpose .