{RDBMS} az postgres flexible-server migration create/show/list/update/check-name-availability; geo-restore; identity assign/remove/show/list; db create/delete; replica list/create/stop-replication/promote; virtual-endpoint create/delete/show/list/update: adding citus on flex validator

{RDBMS} az postgres flexible-server migration create/show/list/update/check-name-availability; geo-restore; identity assign/remove/show/list; db create/delete; replica list/create/stop-replication/promote; virtual-endpoint create/delete/show/list/update: adding citus on flex validator #1890