pgsql: Some refactoring to export json(b) conversion functions

Some refactoring to export json(b) conversion functions

This is to export datum_to_json(), datum_to_jsonb(), and
jsonb_from_cstring(), though the last one is exported as
jsonb_from_text().

A subsequent commit to add new SQL/JSON constructor functions will
need them for calling from the executor.

Discussion: https://postgr.es/m/20230720160252.ldk7jy6jqclxfxkq%40alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b22391a2ff7bdfeff4438f7a9ab26de3e33fdeff

Modified Files
--------------
src/backend/utils/adt/json.c | 59 +++++++++++++++++++++++--------------
src/backend/utils/adt/jsonb.c | 67 +++++++++++++++++++++++++++++++------------
src/include/utils/jsonfuncs.h | 5 ++++
3 files changed, 90 insertions(+), 41 deletions(-)