Postgres connection - add option to set user "role"

**Is your feature request related to a problem? Please describe.**
I am trying to connect my Postgres instance to Cube Core - however our instance is set in a way where in addition to providing Username and Password we also need to provide a user role (https://www.postgresql.org/docs/current/sql-set-role.html)

Based on my research seems like Cube currently does not support this.

It would be great if we could support this as an optional configuration option just like dbt (https://docs.getdbt.com/docs/core/connect-data-platform/postgres-setup)

**Describe the solution you'd like**
It would be great if we could have an environment variable to set user role, just like the ones we have for username and password.
Also, @igorlukanin suggested another option on slack that - we could achieve this via "initSql" config option, just like for it is done in DuckDB
https://github.com/cube-js/cube/blob/34d319a8d81744bde1e655fefeb23496f2fe1089/packages/cubejs-duckdb-driver/src/DuckDBDriver.ts#L163-L173

Thanks