feat: let psql-backup use a URL so `PGSSLMODE=require PGSSLNEGOTIATION=direct` isn't required

[coolaj86]

Currently it's cumbersome to use psql-backup:

PGSSLMODE=require PGSSLNEGOTIATION=direct psql-backup example_db_xxxxxx tls-10-11-12-13.vms.example.com 443

I'd prefer to be able to pass the URL and have it do the right thing - especially if the port is 443.