add a postgres history backend

[slingamn]

First, I had Claude convert the MySQL backend to Postgres: diff

Then I copied and pasted the Postgres backend into a separate package, behind a build tag that builds a stub by default. (As per #1648 the binary size increase from pulling in the postgres driver is very large, about 28%.)

The resulting implementation passes irctest (after patching irctest to talk to postgres).

A least-privileges setup looks like:

user@claudevm:~$ sudo -u postgres psql template1
psql (16.11 (Ubuntu 16.11-0ubuntu0.24.04.1))        
Type "help" for help. 

template1=# ALTER USER postgres with encrypted password 'asdfasdf';
ALTER ROLE
template1=# create user ergo with password 'hunter2';
CREATE ROLE 
template1=# create database ergo_history;        
CREATE DATABASE 
template1=# grant all privileges on database ergo_history to ergo;  
GRANT  
template1=# quit  

user@claudevm:~$ sudo -u postgres psql ergo_history
psql (16.11 (Ubuntu 16.11-0ubuntu0.24.04.1))
Type "help" for help.
ergo_history=# GRANT ALL ON SCHEMA public TO ergo;
GRANT