feat: generate proto-only repository #6802

Job	Run time
build(8) for gapic-generator-java	9m 26s
build(21) except self-service clients	11m 7s
build (11)	12m 27s
build (17)	12m 46s
build(8) except for gapic-generator-java	3m 59s
compatibility	1m 38s
dependency_analyzer_unit_test	16s
gapic-generator-java-bom	1m 46s
lint	20s
showcase (11)	3m 59s
showcase-clirr	3m 0s
showcase (17)	3m 57s
showcase-native	8m 11s
unmanaged_dependency_check	4m 11s
	1h 17m 3s

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