Creating a database with a non-predefined collation

Greetings hackers.

While working on first-class support for PG collations in the Entity
Framework Core ORM, I've come across an interesting problem: it doesn't
seem to be possible to create a database with a collation that isn't
predefined, and there doesn't seem to be a way to add to that list. I'm
specifically looking into creating a database with an ICU collation.

I've seen the discussion in
https://www.postgresql.org/message-id/flat/99faa8eb-9de2-8ec0-0a25-1ad1276167cc%402ndquadrant.com,
though to my understanding, that is about importing ICU rather than libc
predefined collations, and not adding an arbitrary collation to the list
from which databases can be created.

This seems particularly problematic as a database collation cannot be
altered once created, leading to an odd chicken-and-egg problem. My initial
expectation was for collations in the template database to be taken into
account, but that doesn't seem to be the case.

Finally, just a word to say that better support for non-deterministic
collations would be greatly appreciated - specifically LIKE support (though
I realize that isn't trivial). At the moment their actual usefulness seems
somewhat limited because of this.

Thanks,

Shay