Implement `JuMP.copy_extension_data`

```julia
┌ Warning: Model contains extension data of type Dualization.PrimalDualMap{Float64} that we do not know how to copy.
│
│ If you are using a JuMP extension and you did not add data to the `model.ext` dictionary. Please open an issue on the GitHub repository of the JuMP extension and tell them to implement `JuMP.copy_extension_data`.
│
│ If you added things to `model.ext`, they have not been copied.
└ @ JuMP JuMP.jl/src/copy.jl:23
```

It is not obvious to me how to do this nicely since we don't have the `ReferenceMap` in `JuMP.copy_extension_data`. Ideally it would work like

```julia
m = Model()
@variable m x
@constraint m y x ≥ 1
@objective m Min x
dual_model = dualize(model)
copied_dual, ref_map = JuMP.copy_model(dual_model)

# manually propagate keys (current work-around)
y1 = getindex(ref_map, Dualization._get_dual_variables(dual_model, model[:y]))

# proposed behavior -- map from original primal to copied dual
@assert y1 == Dualization._get_dual_variables(copied_dual, model[:y])
```
Alternatively, set it `missing` or something that throws to show the above workaround?