test: add showcase test for api-version #6643

Job	Run time
build (11)	12m 46s
build(21) except self-service clients	11m 49s
build (17)	12m 39s
build(8) except for gapic-generator-java	3m 55s
build(8) for gapic-generator-java	9m 14s
gapic-generator-java-bom	1m 49s
compatibility	1m 29s
lint	36s
showcase (11)	3m 54s
showcase (17)	4m 4s
showcase-native	8m 39s
unmanaged_dependency_check	4m 47s
showcase-clirr	2m 57s
dependency_analyzer_unit_test	17s
	1h 18m 55s

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