Can't create workflow dispatch event by filename

[n1lesh]

Noticed that, GitHub API docs: Create a workflow dispatch event, let's you create dispatch event by both workflowID and Filename.

However, PR #1623 adds support for just workflowID, with CreateWorkflowDispatchEvent.

Can we split it into two funcs for each type, as that's the general implementation in other APIs?

[gmlewis]

Excellent catch, @n1lesh - thank you! Yes, that would be consistent and generally useful.
Let's do that, please.

[gmlewis]

This will be a breaking API change, but I think we should do it, so I've labeled it as such.