Create a separate file listing backend types

Create a separate file listing backend types

Use our established coding pattern to reduce maintenance pain when
adding other per-process-type characteristics.

Like PG_KEYWORD, PG_CMDTAG, PG_RMGR.

To keep the strings translatable, the relevant makefile now also scans
src/include for this specific file.  I didn't want to have it scan all
.h files, as then gettext would have to scan all header files.  I didn't
find any way to affect the meson behavior in this respect though.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Co-authored-by: Jonathan Gonzalez V. <jonathan.abdiel@gmail.com>
Discussion: https://postgr.es/m/202507151830.dwgz5nmmqtdy@alvherre.pgsql