[asyncpg] Extract rowcount for SELECT statements

[asyncpg] Extract rowcount for SELECT statements

Added support to the asyncpg dialect to return the ``cursor.rowcount``
value for SELECT statements when available. While this is not a typical use
for ``cursor.rowcount``, the other PostgreSQL dialects generally provide
this value. Pull request courtesy Michael Gorven.

Fixes: #9048
Closes: #9049
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9049
Pull-request-sha: df16160530c6001d99de059995ad5047a75fb7b0

Change-Id: I095b866779ccea7e4d50bc841fef7605e61c667f
(cherry picked from commit 9c502f5788737fa65029716c73fe0f65f3dafb53)