[client-connection] add a cursor() method for dbapi compatibility
To ease transition from dbapi to repoapi we need the ClientConnection to be as
compatible as possible with the dbapi. Adding this method goes in this
direction.
It'll get deprecated in the deprecation wave that will conclude the repoapi
refactoring.
related to #2503918
[MESSAGES CONTROL]
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[VARIABLES]
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[BASIC]
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[TYPECHECK]
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