pgsql: move hex_decode() to /common so it can be called from frontend

move hex_decode() to /common so it can be called from frontend

This allows removal of a copy of hex_decode() from ecpg, and will be
used by the soon-to-be added pg_alterckey command.

Backpatch-through: master

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c3826f831e6e63e13a749fd3ab9fd7106707b549

Modified Files
--------------
src/backend/utils/adt/encode.c     |  64 +---------------------
src/backend/utils/adt/varlena.c    |   1 +
src/common/Makefile                |   1 +
src/common/hex_decode.c            | 106 +++++++++++++++++++++++++++++++++++++
src/include/common/hex_decode.h    |  16 ++++++
src/include/utils/builtins.h       |   1 -
src/interfaces/ecpg/ecpglib/data.c |  52 +-----------------
src/tools/msvc/Mkvcbuild.pm        |   2 +-
8 files changed, 127 insertions(+), 116 deletions(-)