Support top-level 'let' variable in bulkWrite

This is different than the 'let' in an update statement. The top-level 'let' would should have a "global" scope, whereas 'let' inside a statement would have a local scope. Also see this.

Current 'let' in update op should be renamed to 'constants' and be used to populate setUpdateConstants This parameter should be removed from delete op since it does not support this.

Current 'let' tests should be adapted to use top level let and new tests should be added for update constants.

Top level 'let' should be used to populate setLetParameters in update request and setLet in delete request.

See https://mongodb.slack.com/archives/C04BZMF841J/p1679523866344739 for additional context