Add `GetNamedTimeZoneTransition` method to TimeZoneProvider trait #678

Job	Run time
Test (stable, ubuntu-latest)	27s
Test FFI (stable, ubuntu-latest)	22s
Test (stable, macos-latest)	32s
Test feature matrix	2m 14s
Test (stable, windows-latest)	1m 24s
Documentation	31s
Lint	24s
MSRV	1m 10s
	7m 4s

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