Introduce physical plan version of Operator enum

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The Operator enum is shared between logical and physical plans. I would like to have a separate version for physical plans so that they can start to diverge.

For example, in #3268, I would like to add Expr::IsTrue but map it to the existing binary physical expression with Operator::IsDistinctFrom.

Describe the solution you'd like
☝️

Describe alternatives you've considered
None

Additional context
None