feat(librariangen): add protoc package #12049

Job	Run time
build(21) except self-service clients	13m 11s
build(25) except self-service clients	13m 48s
build(8) except for gapic-generator-java	5m 3s
build (17)	14m 13s
build (11)	14m 50s
build(8) for gapic-generator-java	11m 24s
showcase-clirr	3m 50s
compatibility	2m 21s
showcase (11)	11m 29s
lint	1m 4s
showcase (17)	11m 26s
gapic-generator-java-bom	2m 57s
showcase (25)	11m 30s
build(8) for showcase	5m 8s
unmanaged_dependency_check	9m 28s
dependency_analyzer_unit_test	30s
showcase (21)	11m 44s
	2h 23m 56s

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