feat: add toString to futures returned by operations #7786

Job	Run time
build (11)	15m 2s
build(21) except self-service clients	13m 53s
build (17)	14m 55s
build(8) except for gapic-generator-java	4m 19s
build(8) for gapic-generator-java	12m 19s
compatibility	1m 36s
dependency_analyzer_unit_test	13s
gapic-generator-java-bom	1m 49s
showcase (11)	4m 3s
showcase (17)	4m 5s
lint	25s
showcase (21)	4m 4s
showcase-clirr	3m 8s
showcase-native	8m 6s
unmanaged_dependency_check	4m 21s
	1h 32m 18s

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