Add namespace_exists method #7577

Job	Run time
integration-test	2m 28s
integration-test-gcs	1m 17s
integration-test-s3	1m 13s
lint-and-unit-test (3.12)	1m 0s
lint-and-unit-test (3.13)	1m 1s
lint-and-unit-test (3.10)	59s
lint-and-unit-test (3.11)	1m 1s
integration-test-adls	1m 50s
integration-coverage-report	1s
	10m 50s

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