Allow boxing

[petrnymsa]

Is your feature request related to a problem? Please describe.
We have a common issue where we would like to map from class A to Drift's class B.

But in Drift every field is wrapped in Value object.

Describe the solution you'd like
Allow define boxing of types.

MapType<Source, DriftTarget>(
boxing: BoxingType<Value>(boxing),
),

// somewhere 
Value<T> boxing<T>(T input) => Value(input);