Castable Domains for different JSON representations

Currently domain casts are ignored. Yet this would be very useful for representing data in different formats such as json.

Let's take a tsrange as an example. Its json output by default:

select to_json('(2022-12-31 11:00, 2023-01-01 06:00)'::tsrange);
                       to_json
-----------------------------------------------------
 "(\"2022-12-31 11:00:00\",\"2023-01-01 06:00:00\")"

We can refine its representation in a custom way as:

-- using a custom type for this example

create type mytsrange as range (subtype = timestamp, subtype_diff = tsrange_subdiff);

create or replace function mytsrange_to_json(mytsrange) returns json as $$
  select json_build_object(
    'lower', lower($1)
  , 'upper', upper($1)
  , 'lower_inc', lower_inc($1)
  , 'upper_inc', upper_inc($1)
  );
$$ language sql;

create cast (mytsrange as json) with function mytsrange_to_json(mytsrange) as assignment;

-- now we get the custom representation

select to_json('(2022-12-31 11:00, 2023-01-01 06:00)'::mytsrange);
                                                   to_json
--------------------------------------------------------------------------------------------------------------
 {"lower" : "2022-12-31T11:00:00", "upper" : "2023-01-01T06:00:00", "lower_inc" : false, "upper_inc" : false}
(1 row)

Although this works for this example, using a custom type requires knowledge of the `tsrange` internals. It would be much simpler to do:

create domain mytsrange as range;

But casts on domains are currently ignored: