Make extension types database agnostic (or at least parametric)

[maurer]

Right now, dynamically extended types are implemented via the assumption that they are being implemented for postgres. Since for many types (e.g. bitvectors) it may not be plausible to be completely agnostic, since different databases have different representations, it would instead make sense to be parametric on which database is being used. However, this would cause the Holmes handle itself to need to be parametric over which database is in use, which would be unfortunate.