common/jsonapi: support libpq as a client

common/jsonapi: support libpq as a client

Based on a patch by Michael Paquier.

For libpq, use PQExpBuffer instead of StringInfo. This requires us to
track allocation failures so that we can return JSON_OUT_OF_MEMORY as
needed rather than exit()ing.

Author: Jacob Champion 
Co-authored-by: Michael Paquier 
Co-authored-by: Daniel Gustafsson 
Reviewed-by: Peter Eisentraut 
Discussion: https://www.postgresql.org/message-id/flat/d1b467a78e0e36ed85a09adf979d04cf124a9d4b[email protected]