Function to execute a program

Today we have pg_read_file() and pg_read_binary_file(), which mostly
correspond to COPY from a file, but reads a whole (or parts but based on
bytes) into a single value rather than rows.

Would it make sense to have a pg_execute_program() that corresponds to COPY
FROM PROGRAM? This would obviously have the same permissions restrictions
as COPY FROM PROGRAM.

The usecase would be to for example execute a command that returns json
format output, which could then be parsed and processed as part of a query.
Today, COPY FROM PROGRAM cannot do this, as we can't read a multiline json
value with COPY.

Thoughts?

//Magnus