Add conversions between `Swift.Duration` and `Google_Protobuf_Duration`. #1614

Job	Run time
swift-format Check	33s
Api Breakage Compared to main branch	1m 34s
fuzzing_regressions (debug)	3m 37s
core (6.0.3-noble, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	10m 9s
sanitizer_testing (address, debug)	3m 4s
fuzzing_regressions (release)	3m 24s
core (5.10.1-noble, ref/heads/main)	11m 15s
sanitizer_testing (address, release)	5m 20s
core (5.9.2-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	9m 14s
sanitizer_testing (thread, debug)	3m 25s
core (5.8.1-focal, SWIFT_BUILD_TEST_HOOK="-Xswiftc -warnings-as-errors", ref/heads/main)	8m 12s
sanitizer_testing (thread, release)	4m 53s
	1h 4m 40s

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