feat(execute_prepared_ext): make it possible not to put stmt in session

Alternative to !200

Summary

• fix(execute_prepared_ext):don't put stmt in session

• In picodata SQL we are going to use a separate fiber for doing prepare/unprepare requests, so that when vshard connection is dropped, we don't loose all statements associated with that connection. Current behaviour of execute_prepared_ext is to add prepared stmt to current session, this leads to stmt having 2 refs: one from fiber doing prepare/unprepare, second one from initial fiber executing the statement. This leads to stmt not being deleted from tarantool cache.
• Fix this by not adding stmt to current session

NO_DOC=picodata internal patch NO_CHANGELOG=picodata internal patch NO_TEST=tests will be in picodata MR

Close #... Docs follow-up: not necessary / new issue

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RFC: https://docs.google.com/document/d/181CwxhDwrUkyMbZu7MOUG0BJw2M1rJeuiQP2-M2TpFk