chore: add library generation workflow #6928

Job	Run time
compatibility	1m 52s
build(8) except for gapic-generator-java	4m 23s
build(8) for gapic-generator-java	9m 48s
build(21) except self-service clients	11m 40s
dependency_analyzer_unit_test	28s
build (11)	12m 49s
gapic-generator-java-bom	2m 2s
build (17)	12m 55s
lint	27s
showcase-clirr	2m 59s
showcase (11)	4m 4s
showcase-native	8m 19s
showcase (17)	4m 11s
unmanaged_dependency_check	4m 54s
	1h 20m 51s

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