Added OnlyLValueString test case that shows the problem with current impl. Implemented the change, that accept TimeZonePtrOrName by forwarding reference (to preserve value category), and merged time_zone selection into one trait.
Implemented test for deduction from the custom time zone ptr. Used OffsetZone extracted for that purpose.
Implemented test for deduction from string, string_view, time_zone ptr and other zoned_time.