Add Postgres pure SQL solution

[braco]

This should be added to the documentation IMO, I found it necessary in AWS RDS and was lucky to see it mentioned somewhere:

https://github.com/andreystepanov/hashids.sql

[4kimov]

@braco Thanks for the heads up and sorry for the super late reply. Closing since the project is moving.

@andreystepanov Please let me know if you'd like to help out with https://sqids.org/plpgsql, I'd be happy to add you as a maintainer.