chore: explore parallel library generation #1257

Job	Run time
filter	35s
build(25) except self-service clients	13m 41s
build(8) for gapic-generator-java	11m 4s
build(8) except for gapic-generator-java	3m 40s
showcase-clirr	5m 51s
build(8) for showcase	9m 44s
build (11)	10m 51s
build(21) except self-service clients	13m 5s
build (17)	14m 48s
showcase (11)	14m 43s
dependency_analyzer_unit_test	1m 27s
showcase (17)	13m 10s
unmanaged_dependency_check	12m 52s
showcase (21)	14m 9s
gapic-generator-java-bom	2m 18s
showcase (25)	12m 58s
	2h 34m 56s

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