Add bind casts for BIT on asyncpg

Add bind casts for BIT on asyncpg

Fixed SQL handling for "insertmanyvalues" when using the
:class:`.postgresql.BIT` datatype with the asyncpg backend.  The
:class:`.postgresql.BIT` on asyncpg apparently requires the use of an
asyncpg-specific `BitString` type which is currently exposed when using
this DBAPI, making it incompatible with other PostgreSQL DBAPIs that all
work with plain bitstrings here.  A future fix in version 2.1 will
normalize this datatype across all PG backends.   Pull request courtesy
Sören Oldag.

Fixes: #10532
Closes: #10533
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/10533
Pull-request-sha: 528607d5b5b67c1b9c2edb39176b7e005db371f6

Change-Id: I879f2fc24b5650467b85f170c0def15834f75456