Support SET ROLE for clickhouse.Conn

[jpugliesi]

We have a use case where we'd like to be able to dynamically set the connection ROLE prior to executing a query (and unset it after). Ex:

SET ROLE some_role;
SELECT ...

This does not seem currently possible with the native/clickhouse clickhouse.Conn, because it pools connections under the hood and does not expose an API to retrieve and use the same, stateful connection for multiple statements.

It seems like the only way to achieve this is to use the database/sql driver's BeginTx to execute multiple statements on the same connection.

Is there some way to support SET ROLE with the native/clickhouse driver? If not, are there plans to support this functionality somehow?

This relates to #772, where exposing connection pool's acquire (or hooks into connection acquire/release from the pool) would allow this