chore: migrate sdk-platform-java to monorepo #60

Job	Run time
filter	1m 31s
build(21) except self-service clients	13m 2s
build(25) except self-service clients	12m 57s
build (17)	14m 47s
build(8) for gapic-generator-java	58s
lint	1m 40s
build(8) except for gapic-generator-java	4m 4s
build (11)	14m 38s
compatibility	58s
build(8) for showcase	9m 10s
unmanaged_dependency_check	6m 10s
showcase-clirr	1m 42s
dependency_analyzer_unit_test	1m 13s
showcase (11)	2m 40s
gapic-generator-java-bom	2m 17s
showcase (21)	2m 28s
showcase (17)	2m 35s
showcase (25)	2m 34s
	1h 35m 24s

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