Exposing the underlying driver conn will allow the use of the
standard connection pool while still able to run special function
directly on the driver.

Fixes #29835

Change-Id: Ib6d3b9535e730f008916805ae3bf76e4494c88f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/174182
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>