Allow tailoring of ICU locales with custom rules

Allow tailoring of ICU locales with custom rules

This exposes the ICU facility to add custom collation rules to a
standard collation.

New options are added to CREATE COLLATION, CREATE DATABASE, createdb,
and initdb to set the rules.

Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Daniel Verite <daniel@manitou-mail.org>
Discussion: https://www.postgresql.org/message-id/flat/821c71a4-6ef0-d366-9acf-bb8e367f739f@enterprisedb.com