`CopyConstraintManager` should contain `witness_gen_only`

[jonathanpwang]

Since it is shared everywhere, it should have the source of truth so different structs don't need to individually hold their own trackers of witness_gen_only.

[nyunyunyunyu]

Should CircuitBuilder not have witness_gen_only?

[jonathanpwang]

The problem is we often have different virtual region managers and they each need to know about witness_gen_only. I think we've been encountering a bunch of places where it's inconvenient to also pass it in or make sure CircuitBuilder is always accessible?