feat: generate proto-only repository #6654

Job	Run time
build(8) except for gapic-generator-java	4m 7s
build (11)	12m 16s
build (17)	12m 34s
build(21) except self-service clients	11m 11s
build(8) for gapic-generator-java	9m 17s
compatibility	1m 36s
gapic-generator-java-bom	1m 45s
lint	26s
showcase (11)	4m 14s
showcase-clirr	2m 58s
showcase (17)	4m 18s
showcase-native	7m 58s
dependency_analyzer_unit_test	16s
unmanaged_dependency_check	3m 51s
	1h 16m 47s

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