deps: Make opentelemetry-api an optional dependency. #6267

Job	Run time
build (11)	12m 13s
build (17)	12m 21s
build(8) except for gapic-generator-java	3m 54s
build(21) except self-service clients	11m 11s
build(8) for gapic-generator-java	9m 23s
compatibility	1m 36s
dependency_analyzer_unit_test	23s
gapic-generator-java-bom	1m 51s
lint	23s
showcase (11)	3m 48s
showcase (17)	4m 6s
showcase-clirr	3m 1s
showcase-native	8m 3s
unmanaged_dependency_check	5m 49s
	1h 18m 2s

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