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

Bruce Momjian <bruce@momjian.us>

Commit: c3826f831e6e63e13a749fd3ab9fd7106707b549
Author: Bruce Momjian <bruce@momjian.us>
Date: 2020-12-24T22:25:48Z
Releases: 14.0
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

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