Allow to customise units where prefix k means 1024 instead of 1000

Zabbix has a hardcoded list of units where e.g. the prefix "k" means 1024 instead of 1000.
Currently this includes "B" and "Bps".

It would be useful to allow customisation of that list. In german you could e.g. use "B/s" instead of Bps or even write "Bytes".

This is closely related to ZBXNEXT-768.
Please note that there are also other aproaches to this problem: ZBX-4390, ZBXNEXT-954.
and a specification page: https://zabbix.org/wiki/Docs/specs/ZBX-4390.