chore: add generation workflow #6935

Job	Run time
build (11)	12m 38s
build (17)	12m 43s
build(21) except self-service clients	11m 48s
build(8) except for gapic-generator-java	4m 15s
compatibility	1m 57s
lint	23s
build(8) for gapic-generator-java	9m 17s
dependency_analyzer_unit_test	23s
gapic-generator-java-bom	1m 58s
showcase (11)	4m 0s
showcase (17)	4m 8s
showcase-clirr	3m 16s
showcase-native	8m 14s
unmanaged_dependency_check	4m 37s
	1h 19m 37s

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