Add conversions between `Swift.Duration` and `Google_Protobuf_Duration`. #1613

Job	Run time
Api Breakage Compared to main branch	1m 36s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	15m 39s
swift-format Check	35s
core (5.10.1-noble, ref/heads/main)	16m 11s
fuzzing_regressions (debug)	3m 43s
sanitizer_testing (address, debug)	2m 37s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	15m 14s
fuzzing_regressions (release)	3m 7s
sanitizer_testing (address, release)	5m 21s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	14m 3s
sanitizer_testing (thread, debug)	2m 48s
sanitizer_testing (thread, release)	4m 48s
	1h 25m 42s

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