Use Factory pattern for BatchSignUtil

BatchSignInput property was added to SignToolTask so that it could be exposed during unit testing validation.  A cleaner solution would be to use the factory pattern and mock out BatchSignUtil so that the BatchSignInput values could be validated.

See https://github.com/dotnet/arcade/pull/6746#discussion_r552926462