The custom module provides a parametric wrapper, allowing any specific
FromJSON/ToJSON instance to be used. It's a standalone module though, and not a
wrapper of persistent-postgresql, because persistent-postgresql uses aeson
Value and it prevents using toEncoding to get from the value directly to a
string.
Using a dedicated type allows to record in the type the guarantees that we
provide, such as scheme being HTTPS and authority being present. Allows to
replace ugly `fromJust` and such with direct field access.
