Postgres extension support schema

## Help us help you
  - [x] I have checked that my issue [doesn't exist yet](https://github.com/alliedmodders/sourcemod/issues).

## Is your feature request related to a problem? Please describe.
I want to put my plugin in a schema which is not `public`

## Describe the solution you'd like
Add an option `schema` in database config, connect to a specific schema