Add `UtcOffset` struct for `PartialZonedDateTime` #683

Job	Run time
Documentation	31s
Test (stable, ubuntu-latest)	22s
Test FFI (stable, ubuntu-latest)	17s
Test (stable, macos-latest)	21s
Test feature matrix	19s
Lint	20s
Test (stable, windows-latest)	31s
MSRV	1m 9s
	3m 50s

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