Enable using UTC timestamps instead of local

[igor-sirotin]

Currently timestamps use now() which means local timezone:

nim-chronicles/chronicles/log_output.nim

Lines 425 to 426 in ccbb756

	proc rfcTimestamp: string =
	now().format("yyyy-MM-dd HH:mm:ss'.'fffzzz")

It's not convenient when you're trying to analyse logs for users from different timezones.
In this case it's common to use UTC timestamps.

This is an feature that would be nice to have in status-desktop app:

• Logs in UTC time status-desktop#15198