Support UUID as PKs in associations

[solnic]

This adds support for PKs generated by the factory. A common example is using UUIDs as PKs.

ROM.container(:sql, conn) do |conf|
  conf.default.create_table(:workers) do
    column :id, :uuid, primary_key: true
    column :name, String
  end

  conf.default.create_table(:jobs) do
    column :id, :uuid, primary_key: true
    foreign_key :worker_id, :workers, type: :uuid
    column :name, String
  end

  conf.relation(:workers) do
    schema(infer: true) do
      associations do
        has_many :jobs
      end
    end
  end

  conf.relation(:jobs) do
    schema(infer: true) do
      attribute :worker_id, ROM::SQL::Types.ForeignKey(:workers, ROM::SQL::Types::String)

      associations do
        belongs_to :worker
      end
    end
  end
end

factories.define(:worker) do |f|
  f.id { fake(:internet, :uuid) }
  f.name "Test Worker"

  f.association(:jobs, count: 0)

  f.trait(:with_jobs) do |t|
    t.association(:jobs, count: 2)
  end
end

factories.define(:job) do |f|
  f.id { fake(:internet, :uuid) }
  f.name "Test Job"

  f.association :worker
end

Bonus

This also improves returning persisted structs when RETURNING can't work as expected (ie in Sqlite or in case of PKs generated on the Ruby side).