feat(gax): add API key authentication to ClientSettings #8467

Job	Run time
build (11)	14m 46s
build(21) except self-service clients	13m 40s
build (17)	14m 34s
build(8) for gapic-generator-java	11m 41s
compatibility	1m 29s
lint	23s
build(8) except for gapic-generator-java	4m 26s
dependency_analyzer_unit_test	17s
gapic-generator-java-bom	1m 43s
showcase (11)	4m 1s
showcase (17)	4m 18s
showcase-clirr	3m 6s
showcase (21)	4m 14s
showcase-native	7m 59s
unmanaged_dependency_check	4m 16s
	1h 30m 53s

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