feat: generate proto-only repository #6653

Job	Run time
build(8) except for gapic-generator-java	3m 50s
build (11)	12m 35s
build(8) for gapic-generator-java	9m 37s
build (17)	12m 19s
lint	25s
build(21) except self-service clients	10m 53s
compatibility	1m 27s
dependency_analyzer_unit_test	17s
gapic-generator-java-bom	1m 49s
showcase (11)	3m 53s
showcase (17)	4m 3s
showcase-clirr	2m 56s
showcase-native	8m 1s
unmanaged_dependency_check	5m 38s
	1h 17m 43s

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