Enhancement: Support adding attribute to methods

[IeuanWalker]

At the moment attributes can only be added to a class with an Execute() method implemented within it.

Update the source generator to also work directly on a method.

[IeuanWalker]

It's not something I'd personally do as I believe recurring jobs should be contained in it's own class.
But if I have time I'll look into implementing/ updating the code to also support methods.

The way the code is setup it should be pretty easy for someone to create a PR to add support for it, so feel free to give it a shot if you want,