Improve WorkflowOptionBuilderExtension

It's a bit odd to have a separate class for extending WorkflowOption.
I don't remember why we can't just put it under WorkflowOption. Just a note here to think through it later

Start by locating WorkflowOptionBuilderExtension and WorkflowOption and comparing their responsibilities and usage across the SDK. The issue does not identify files, tests, or a concrete desired outcome; done would require deciding whether the extension should be consolidated and verifying the resulting design with the project maintainers.

Written by the indexing model from the issue text.