chore: demo offset parsing tests in java-core + java.time #8966

Job	Run time
build (11)	3m 12s
build (17)	3m 8s
build(21) except self-service clients	3m 17s
build(8) except for gapic-generator-java	3m 38s
build(8) for gapic-generator-java	11m 23s
compatibility	1m 34s
lint	26s
showcase (11)	4m 2s
showcase (17)	4m 5s
dependency_analyzer_unit_test	22s
showcase (21)	4m 0s
gapic-generator-java-bom	1m 49s
showcase-clirr	3m 7s
showcase-native	8m 9s
unmanaged_dependency_check	4m 29s
	56m 41s

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