Add namespace_exists method #7579

Job	Run time
integration-test-adls	1m 26s
integration-test-s3	1m 22s
lint-and-unit-test (3.11)	2m 45s
lint-and-unit-test (3.13)	3m 2s
integration-test	10m 9s
lint-and-unit-test (3.10)	2m 54s
integration-test-gcs	1m 10s
lint-and-unit-test (3.12)	2m 59s
integration-coverage-report	35s
	26m 22s

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