chore: migrate sdk-platform-java to monorepo #88

Job	Run time
filter	1m 14s
build(8) for gapic-generator-java	2m 28s
build(21) except self-service clients	5m 27s
build(25) except self-service clients	4m 51s
build(8) for showcase	9m 46s
gapic-generator-java-bom	3m 41s
build(8) except for gapic-generator-java	4m 12s
dependency_analyzer_unit_test	1m 18s
build (17)	5m 12s
showcase-clirr	1m 43s
unmanaged_dependency_check	9m 20s
showcase (25)	10m 7s
build (11)	5m 15s
showcase (11)	10m 9s
showcase (17)	9m 32s
showcase (21)	9m 41s
	1h 33m 56s

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