Add support for optional CancellationToken parameters

This adds 28 new async methods with optional CancellationToken parameters support.

The methods without CancellationToken have been preserved in the `SqlMapper.Async.Legacy.cs` file for binary backward compatibility. The `this` modifier has been removed from those methods in order to also preserve source compatibility.

This pull request supersedes DapperLib#2138

Author: 0xced