Add support for preserve_rowcount execution_option

Add support for preserve_rowcount execution_option

Added new core execution option
paramref:`_engine.Connection.execution_options.preserve_rowcount`
to unconditionally save the ``rowcount`` attribute from the cursor in the
class:`_engine.Result` returned from an execution, regardless of the
statement being executed.
When this option is provided the correct value is also set when
an INSERT makes use of the "insertmanyvalues" mode, that may use
more than one actualy cursor execution.

Fixes: #10974
Change-Id: Icecef6b7539be9f0a1a02b9539864f5f163dcfbc