pgsql: Add HINT listing valid encodings to encode() and decode() errors

Add HINT listing valid encodings to encode() and decode() errors.

This commit updates encode() and decode() so that when an invalid encoding
is specified, their error message includes a HINT listing all valid encodings.
This helps users quickly see which encodings are supported without needing
to consult the documentation.

Author: Shinya Sugamoto <shinya34892(at)gmail(dot)com>
Reviewed-by: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Reviewed-by: Nathan Bossart <nathandbossart(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAe3y+99sfPv8UDF1VM-rC1i5HBdqxUh=2HrbJJFm2+i=1OwOw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/99780da7209605bf9f226eac3eac30ab2bc9427c

Modified Files
--------------
src/backend/utils/adt/encode.c | 8 ++++++--
src/test/regress/expected/strings.out | 7 +++++++
src/test/regress/sql/strings.sql | 4 ++++
3 files changed, 17 insertions(+), 2 deletions(-)