support setting a role on a per-query basis

[svix-jbrown]

Use case

The HTTP interface supports setting a role on a per-query basis. It would be nice to expose this in clickhouse-rs.

Describe the solution you'd like

Another method on clickhouse::Client called .with_role() which will cause all subsequent queries issued by that Client to have the role=... kwarg

Also, a .with_role() arg on Query.

Describe the alternatives you've considered

This could be hacked in with .with_options(), but I think it's clearer to have it as a dedicated method since roles are so central to authorization.

Additional context