Add a retry_aborts_internally flag to the DBAPI Connection class and
the connect() function. When set to False, aborted transactions raise
RetryAborted directly from commit() instead of entering the internal
statement-replay retry loop.

This aligns with RETRY_ABORTS_INTERNALLY in the Spanner JDBC driver
and avoids nested retry loops when the application manages its own
transaction retry logic.

Fixes googleapis#16491