pgbouncer: Add auth_dbname option

[vitabaks]

The database name “pgbouncer” is reserved for the admin console.

Related MR: #401

Fixed:

postgres@pgnode02:~$ psql -h 127.0.0.1 -p 6432 -U postgres -d pgbouncer
psql: error: connection to server at "127.0.0.1", port 6432 failed: FATAL:  bouncer config error

pgbouncer.log

2023-08-03 12:46:22.405 UTC [15102] ERROR C-0x562de93bce50: (nodb)/(nouser)@127.0.0.1:55328 cannot use the reserved "pgbouncer" database as an auth_dbname
2023-08-03 12:46:22.405 UTC [15102] WARNING C-0x562de93bce50: (nodb)/(nouser)@127.0.0.1:55328 pooler error: bouncer config error
2023-08-03 12:46:24.158 UTC [15102] LOG stats: 0 xacts/s, 0 queries/s, in 0 B/s, out 0 B/s, xact 0 us, query 0 us, wait 0 us