feat: Explict Configuration #8

Job	Run time
build (11)	11m 36s
build(8) for gapic-generator-java	13m 33s
build (17)	21m 34s
build(8) except for gapic-generator-java	4m 51s
compatibility	1m 30s
gapic-generator-java-bom	2m 28s
lint	6h 0m 2s
requirements-kokoro	1m 18s
showcase (11)	4m 26s
showcase-clirr	3m 59s
showcase (17)	4m 25s
showcase-native	4m 1s
	7h 13m 43s

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