Support for SASL-SCRAM-256

[adfoster-r7]

Hi there

I've recently added SASL-SCRAM-256 support to a project that uses and vendors the postgres-pr dependency; I was wondering if it's worth upstreaming these changes directly into this main gem as well? Happy to hear your thoughts

Code in question: rapid7/metasploit-framework#18276

[malcolm-pro]

These seems worth pursuing, as SCRAM is the new default in Postgres; maybe a PR would be worth opening?