chore: migrate sdk-platform-java to monorepo #70

Job	Run time
filter	1m 19s
build(8) except for gapic-generator-java	3m 54s
build(21) except self-service clients	13m 36s
build(25) except self-service clients	12m 30s
build(8) for showcase	9m 14s
build(8) for gapic-generator-java	11m 27s
showcase (25)	5m 35s
dependency_analyzer_unit_test	1m 18s
showcase (17)	5m 28s
build (17)	13m 36s
unmanaged_dependency_check	11m 3s
showcase (21)	5m 18s
gapic-generator-java-bom	2m 38s
showcase (11)	5m 29s
showcase-clirr	1m 33s
build (11)	13m 51s
	1h 57m 49s

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