sql/postgres: add sequence to sqlspec #1006

Job	Run time
integration-maria107	1m 26s
integration-mysql56	1m 24s
integration-maria103	1m 22s
integration-mysql57	1m 26s
integration-mysql8	1m 40s
integration-maria102	1m 29s
integration-postgres10	3m 55s
integration-postgres11	1m 47s
integration-postgres13	1m 40s
integration-postgres12	1m 39s
integration-postgres14	1m 47s
integration-postgres15	3m 55s
integration-postgres-ext-postgis	4m 9s
integration-sqlite	1m 9s
integration-cockroach	1m 21s
integration-tidb5	1m 57s
integration-tidb6	1m 55s
	34m 1s

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