Enable direct conversion from configuration parameter to `product_query`

[knoepfel]

Users should be able to specify a configuration like

module_name: {
  plugin: "<plugin path>",
  input_product: { product: "a", layer: "spill"}
}

and then, in the registration code, be able to directly express the value of the input_product parameter above as a product_query object:

PHLEX_REGISTER_ALGORITHMS(m, config)
{
  m.transform(...).input_family(config.get<product_query>("input_product")); 

  // which is equivalent to:
  //
  // m.transform(...).input_family(
  //   product_query{config.get<std::string>("input_product.product"),
  //                 config.get<std::string>("input_product.player")}
  // );
}

---

It would be desirable to use the in: parameter instead of layer:. However, because in is a keyword in Jsonnet, it would need to be surrounded by quotes (e.g., "in":). To avoid this awkwardness, we use layer instead.