Support both sqlite and postgres in ash application

I got it working by doing:

  if App.Config.use_sqlite?() do
    import AshSqlite.DataLayer

    sqlite do
      # ...
    end
  else
    import AshPostgres.DataLayer

    postgres do
      # ...
    end
  end

A bit weird, since the use MyApp.Resource should already have imported those definitions, at least for the branch that is compiled.

Anyway, that seems to work and isn’t too bad..