[v1] Support operator node within AST

[alancai98]

Similar to #1460, we can consider simplifying our AST representation of many unary and binary operators into a dedicated operator node. This can allow us to eventually support custom operators similar to what postgresql allows users to define -- https://www.postgresql.org/docs/current/sql-createoperator.html.

This issue does not track actually plumbing through operator resolution into the SPI and catalog but gets rid of some AST nodes that can be modeled with the operator node.