pgsql: Add base32hex support to encode() and decode() functions.

Add base32hex support to encode() and decode() functions.

This adds support for base32hex encoding and decoding, as defined in
RFC 4648 Section 7. Unlike standard base32, base32hex uses the
extended hex alphabet (0-9, A-V) which preserves the lexicographical
order of the encoded data.

This is particularly useful for representing UUIDv7 values in a
compact string format while maintaining their time-ordered sort
property.

The encode() function produces output padded with '=', while decode()
accepts both padded and unpadded input. Following the behavior of
other encoding types, decoding is case-insensitive.

Suggested-by: Sergey Prokhorenko <sergeyprokhorenko@yahoo.com.au>
Author: Andrey Borodin <x4mmm@yandex-team.ru>
Co-authored-by: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Илья Чердаков <i.cherdakov.pg@gmail.com>
Reviewed-by: Chengxi Sun <chengxisun92@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAJ7c6TOramr1UTLcyB128LWMqita1Y7%3Darq3KHaU%3Dqikf5yKOQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/497c1170cb108cd84fe13db19c049177376fec5f

Modified Files
--------------
doc/src/sgml/func/func-binarystring.sgml |  27 ++++++
src/backend/utils/adt/encode.c           | 161 +++++++++++++++++++++++++++++-
src/test/regress/expected/strings.out    | 162 ++++++++++++++++++++++++++++++-
src/test/regress/expected/uuid.out       |  18 +++-
src/test/regress/sql/strings.sql         |  57 ++++++++++-
src/test/regress/sql/uuid.sql            |   8 +-
6 files changed, 420 insertions(+), 13 deletions(-)