Add support for multiple database connections & roles for PG via connection string

[crimson-knight]

While adding automatic connection switching, @robacarp pointed out that PG supports this: https://github.com/amberframework/granite/pull/476/files#r1191411657

https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNECT-TARGET-SESSION-ATTRS

This would be a great enhancement :)