Feature request: Multi host support with target_session_attrs option

It would be really cool to allow multiple hosts like [with psql/libpq](https://www.postgresql.org/docs/13/libpq-connect.html#LIBPQ-MULTIPLE-HOSTS).

That way we could have connections to HA setups where we use `target_session_attrs` to define either connecting to a read-write host or a read-only host.

No files or tests are named. Start with the linked PostgreSQL libpq multiple-host documentation and locate the existing connection setup entry point; done means supporting multiple hosts and the target_session_attrs choice for read-write or read-only connections.

Written by the indexing model from the issue text.