feat: add entry point #5971

Job	Run time
build (11)	12m 18s
build(21) except self-service clients	11m 0s
build (17)	12m 27s
build(8) except for gapic-generator-java	3m 54s
build(8) for gapic-generator-java	9m 26s
compatibility	1m 14s
lint	28s
gapic-generator-java-bom	1m 26s
showcase (11)	4m 7s
showcase-clirr	2m 48s
showcase (17)	4m 13s
showcase-native	8m 12s
unmanaged_dependency_check	4m 42s
	1h 16m 15s

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