Add namespace_exists method #7578

Job	Run time
integration-test	11m 9s
integration-test-gcs	1m 17s
lint-and-unit-test (3.12)	3m 5s
lint-and-unit-test (3.10)	2m 53s
integration-test-s3	1m 10s
lint-and-unit-test (3.11)	2m 48s
lint-and-unit-test (3.13)	3m 0s
integration-test-adls	1m 24s
integration-coverage-report	28s
	27m 14s

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